Android getActivity从片段返回null

我正在为学校制作一款android应用程序。对于这个应用程序，我有一个片段，它显示了一个数据库中的gridview，其中只包含字符串。为此，我需要一个片段。调用getActivity（）时，它返回null。在应用程序崩溃之前，不会调用建议的onAttach方法。我应该如何解决这个问题？
weekFragmetn.xml：

package nl.siebeh.schoolmate;

import java.util.ArrayList;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.ContextMenu;
import android.view.ContextMenu.ContextMenuInfo;
import android.view.LayoutInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.GridView;

public class weekFragment extends Fragment {
    dbLayer db;
    Context mContext;

    @Override
    public void onAttach(Activity activity) {
        mContext = getActivity();
        Log.i("Schoolmate","onAttach called");
        super.onAttach(activity);
    }

    public weekFragment newInstance(String title) {
        Log.i("Schoolmate","newInstance called");
        db = new dbLayer(mContext);
        weekFragment fragment = new weekFragment();
        ArrayList<ArrayList<Object>> result = null;

        if(title == "leraren"){
            result = db.getAllRowsAsArrays("weekTeacher", null);
        }else if(title == "locatie"){
            result = db.getAllRowsAsArrays("weekLocation", null);
        }else if(title == "vakken"){
            result = db.getAllRowsAsArrays("weekSubjects", null);
        }

        String[] array = new String[60];

        for(int i = 0; i < 60; i++){
            array[i] = "";
        }

        array[1] = getResources().getStringArray(R.array.days)[0];
        array[2] = getResources().getStringArray(R.array.days)[1];
        array[3] = getResources().getStringArray(R.array.days)[2];
        array[4] = getResources().getStringArray(R.array.days)[3];
        array[5] = getResources().getStringArray(R.array.days)[4];

        for(int i = 1; i < 10; i++){
            array[i] = Integer.toString(i);
        }

        for(int position = 0; position < result.size(); position++){
            ArrayList<Object> row = result.get(position);
            int hour = Integer.valueOf(row.get(1).toString()).intValue();
            int day =  Integer.valueOf(row.get(0).toString()).intValue();
            int pos = 6 * hour + day;
            array[pos] = row.get(position).toString();
        }
        fragment.mContent = array;

        return fragment;
    }

    private String[] mContent = {""};

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        if (container == null) {
            return null;
        }
        View v = inflater.inflate(R.layout.week_fragment, container, false);
        GridView gridview = (GridView) v.findViewById(R.id.gridview);
        gridview.setAdapter(new weekAdapter(mContext, mContent));
        return gridview;
    }

    @Override
    public boolean onContextItemSelected(MenuItem item) {
        return super.onContextItemSelected(item);
    }

    @Override
    public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {
        super.onCreateContextMenu(menu, v, menuInfo);
    }



}

包nl.siebeh.school；
导入java.util.ArrayList；
导入android.app.Activity；
导入android.content.Context；
导入android.os.Bundle；
导入android.support.v4.app.Fragment；
导入android.util.Log；
导入android.view.ContextMenu；
导入android.view.ContextMenu.ContextMenuInfo；
导入android.view.LayoutInflater；
导入android.view.MenuItem；
导入android.view.view；
导入android.view.ViewGroup；
导入android.widget.GridView；
公共类weekFragment扩展了片段{
数据库层数据库；
语境；
@凌驾
公共事务主任（活动）{
mContext=getActivity（）；
Log.i（“同学”、“打电话来”）；
超级转速计（活动）；
}
public weekFragment newInstance（字符串标题）{
Log.i（“同学”、“新实例”）；
db=新的dbLayer（mContext）；
weekFragment片段=新的weekFragment（）；
ArrayList结果=null；
如果（标题=“Lerren”）{
结果=db.getAllrowsaarrays（“weekTeacher”，null）；
}否则如果（标题=“位置”）{
结果=db.getAllRowsAsArray（“周定位”，null）；
}否则，如果（标题=“vakken”）{
结果=db.getAllRowsAsArray（“周子对象”，null）；
}
字符串[]数组=新字符串[60]；
对于（int i=0；i<60；i++）{
数组[i]=“”；
}
数组[1]=getResources（）.getStringArray（R.array.days）[0]；
数组[2]=getResources（）.getStringArray（R.array.days）[1]；
数组[3]=getResources（）.getStringArray（R.array.days）[2]；
数组[4]=getResources（）.getStringArray（R.array.days）[3]；
数组[5]=getResources（）.getStringArray（R.array.days）[4]；
对于（int i=1；i<10；i++）{
数组[i]=整数.toString（i）；
}
对于（int position=0；position

logcat：（因为单击了启动活动的列表中的项目，片段在其中启动/嵌入）

09-29 19:10:56.611:INFO/ActivityManager（70）：从pid 536开始：Intent{cmp=nl.siebeh.school mate/.overviewWeek}
09-29 19:10:56.970:INFO/同学（536）：新实例呼叫
09-29 19:11:06.631:警告/活动管理器（70）：启动超时已过期，放弃唤醒锁定！
09-29 19:11:06.671:WARN/ActivityManager（70）：历史记录的活动空闲超时{40770188 nl.siebeh.schoolmate/.overviewWeek}
09-29 19:12:20.661:DEBUG/SntpClient（70）：请求时间失败：java.net.SocketException:协议不支持地址系列
09-29 19:14:11.811:DEBUG/dalvikvm（536）：调试器已分离；对象注册表有413个条目
09-29 19:14:11.811:错误/同学（536）：java.lang.NullPointerException
09-29 19:14:11.811:信息/同学（536）：上下文为空
09-29 19:14:11.820:WARN/System.err（536）：java.lang.NullPointerException
09-29 19:14:11.820:WARN/System.err（536）：位于android.database.sqlite.SQLiteOpenHelper.getwriteabledatabase（SQLiteOpenHelper.java:118）
09-29 19:14:11.820:WARN/System.err（536）：在nl.siebeh.school.dbLayer.（dbLayer.java:50）
09-29 19:14:11.820:WARN/System.err（536）：在nl.siebeh.schoolmate.weekFragment.newInstance（weekFragment.java:31）
09-29 19:14:11.820:WARN/System.err（536）：在nl.siebeh.schoolmate.fragmentAdapter.getItem（fragmentAdapter.java:21）
09-29 19:14:11.820:WARN/System.err（536）：在nl.siebeh.school.titleFragmentAdapter.getItem（titleFragmentAdapter.java:1）
09-29 19:14:11.820:WARN/System.err（536）：在android.support.v4.app.FragmentPagerAdapter.instanceItem（FragmentPagerAdapter.java:62）
09-29 19:14:11.820:WARN/System.err（536）：位于android.support.v4.view.ViewPager.addNewItem（ViewPager.java:321）
09-29 19:14:11.820:WARN/System.err（536）：在android.support.v4.view.ViewPager.populate（ViewPager.java:441）
09-29 19:14:11.820:WARN/System.err（536）：在android.support.v4.view.ViewPager.onAttachedToWindow（ViewPager.java:563）
09-29 19:14:11.820:WARN/System.err（536）：在android.view.view.dispatchAttachedToWindow（view.java:6156）
09-29 19:14:11.820:WARN/System.err（536）：在android.view.ViewGroup.dispatchAttachedToWindow（ViewGroup.java:1122）
09-29 19:14:11.820:WARN/System.err（536）：在android.view.ViewGroup.dispatchAttachedToWindow（ViewGroup.java:1127）
09-29 19:14:11.820:WARN/System.err（536）：在android.view.ViewGroup.dispatchAttachedToWindow（ViewGroup.java:1127）
09-29 19:14:11.820:WARN/System.err（536）：在android.view.ViewGroup.dispatchAttachedToWindow（ViewGroup）上
    09-29 19:10:56.611: INFO/ActivityManager(70): Starting: Intent { cmp=nl.siebeh.schoolmate/.overviewWeek } from pid 536
09-29 19:10:56.970: INFO/Schoolmate(536): newInstance called
09-29 19:11:06.631: WARN/ActivityManager(70): Launch timeout has expired, giving up wake lock!
09-29 19:11:06.671: WARN/ActivityManager(70): Activity idle timeout for HistoryRecord{40770188 nl.siebeh.schoolmate/.overviewWeek}
09-29 19:12:20.661: DEBUG/SntpClient(70): request time failed: java.net.SocketException: Address family not supported by protocol
09-29 19:14:11.811: DEBUG/dalvikvm(536): Debugger has detached; object registry had 413 entries
09-29 19:14:11.811: ERROR/SchoolMate(536): java.lang.NullPointerException
09-29 19:14:11.811: INFO/SchoolMate(536): Context is null
09-29 19:14:11.820: WARN/System.err(536): java.lang.NullPointerException
09-29 19:14:11.820: WARN/System.err(536):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:118)
09-29 19:14:11.820: WARN/System.err(536):     at nl.siebeh.schoolmate.dbLayer.<init>(dbLayer.java:50)
09-29 19:14:11.820: WARN/System.err(536):     at nl.siebeh.schoolmate.weekFragment.newInstance(weekFragment.java:31)
09-29 19:14:11.820: WARN/System.err(536):     at nl.siebeh.schoolmate.fragmentAdapter.getItem(fragmentAdaptar.java:21)
09-29 19:14:11.820: WARN/System.err(536):     at nl.siebeh.schoolmate.titleFragmentAdapter.getItem(titleFragmentAdapter.java:1)
09-29 19:14:11.820: WARN/System.err(536):     at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:62)
09-29 19:14:11.820: WARN/System.err(536):     at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:321)
09-29 19:14:11.820: WARN/System.err(536):     at android.support.v4.view.ViewPager.populate(ViewPager.java:441)
09-29 19:14:11.820: WARN/System.err(536):     at android.support.v4.view.ViewPager.onAttachedToWindow(ViewPager.java:563)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.View.dispatchAttachedToWindow(View.java:6156)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1122)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.820: WARN/System.err(536):     at android.view.ViewRoot.performTraversals(ViewRoot.java:765)
09-29 19:14:11.830: WARN/System.err(536):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1859)
09-29 19:14:11.840: WARN/System.err(536):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-29 19:14:11.840: WARN/System.err(536):     at android.os.Looper.loop(Looper.java:123)
09-29 19:14:11.840: WARN/System.err(536):     at android.app.ActivityThread.main(ActivityThread.java:3683)
09-29 19:14:11.840: WARN/System.err(536):     at java.lang.reflect.Method.invokeNative(Native Method)
09-29 19:14:11.840: WARN/System.err(536):     at java.lang.reflect.Method.invoke(Method.java:507)
09-29 19:14:11.850: WARN/System.err(536):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
09-29 19:14:11.850: WARN/System.err(536):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
09-29 19:14:11.850: WARN/System.err(536):     at dalvik.system.NativeStart.main(Native Method)
09-29 19:14:11.850: DEBUG/AndroidRuntime(536): Shutting down VM
09-29 19:14:11.850: WARN/dalvikvm(536): threadid=1: thread exiting with uncaught exception (group=0x40015560)
09-29 19:14:11.870: ERROR/AndroidRuntime(536): FATAL EXCEPTION: main
09-29 19:14:11.870: ERROR/AndroidRuntime(536): java.lang.NullPointerException
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at nl.siebeh.schoolmate.weekFragment.newInstance(weekFragment.java:49)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at nl.siebeh.schoolmate.fragmentAdapter.getItem(fragmentAdaptar.java:21)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at nl.siebeh.schoolmate.titleFragmentAdapter.getItem(titleFragmentAdapter.java:1)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:62)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:321)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.support.v4.view.ViewPager.populate(ViewPager.java:441)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.support.v4.view.ViewPager.onAttachedToWindow(ViewPager.java:563)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.View.dispatchAttachedToWindow(View.java:6156)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1122)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewGroup.dispatchAttachedToWindow(ViewGroup.java:1127)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewRoot.performTraversals(ViewRoot.java:765)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1859)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.os.Looper.loop(Looper.java:123)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at android.app.ActivityThread.main(ActivityThread.java:3683)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at java.lang.reflect.Method.invokeNative(Native Method)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at java.lang.reflect.Method.invoke(Method.java:507)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
09-29 19:14:11.870: ERROR/AndroidRuntime(536):     at dalvik.system.NativeStart.main(Native Method)
09-29 19:14:11.890: WARN/ActivityManager(70):   Force finishing activity nl.siebeh.schoolmate/.overviewWeek
09-29 19:14:12.413: WARN/ActivityManager(70): Activity pause timeout for HistoryRecord{40770188 nl.siebeh.schoolmate/.overviewWeek}
09-29 19:14:21.910: WARN/ActivityManager(70): Launch timeout has expired, giving up wake lock!
09-29 19:14:22.441: WARN/ActivityManager(70): Activity idle timeout for HistoryRecord{4070c7a0 nl.siebeh.schoolmate/.schoolmate}
09-29 19:14:32.451: WARN/ActivityManager(70): Activity destroy timeout for HistoryRecord{40770188 nl.siebeh.schoolmate/.overviewWeek}