android wifi状态更改导致应用程序崩溃

考虑到我的这个问题，我仍然无法找到为什么wifi状态更改会导致应用程序崩溃，当我的应用程序处于重新启动状态时从启动器返回，它会在恢复时更改为oncreate，然后应用程序崩溃，我试图在第二次活动的顶部在SharedRef中保存一个id，但仍然崩溃。
这里是logcat：

12-05 17:14:39.966: E/AndroidRuntime(29449): FATAL EXCEPTION: main
12-05 17:14:39.966: E/AndroidRuntime(29449): java.lang.RuntimeException: Unable to           start activity     ComponentInfo{com.Pakagename/com.Pakagename.subCategories.SubCategoriesActivity}:     java.lang.NullPointerException
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2205)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2240)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.app.ActivityThread.access$600(ActivityThread.java:139)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1262)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.os.Handler.dispatchMessage(Handler.java:99)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.os.Looper.loop(Looper.java:156)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.app.ActivityThread.main(ActivityThread.java:4977)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at java.lang.reflect.Method.invokeNative(Native Method)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at java.lang.reflect.Method.invoke(Method.java:511)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at dalvik.system.NativeStart.main(Native Method)
12-05 17:14:39.966: E/AndroidRuntime(29449): Caused by: java.lang.NullPointerException
12-05 17:14:39.966: E/AndroidRuntime(29449):    at com.Pakagename.subCategories.SubCategoriesActivity.getSubCategories(SubCategoriesActivity.java:119)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at com.Pakagename.subCategories.SubCategoriesActivity.onCreate(SubCategoriesActivity.java:90)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.app.Activity.performCreate(Activity.java:4538)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1071)
12-05 17:14:39.966: E/AndroidRuntime(29449):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2161)
12-05 17:14:39.966: E/AndroidRuntime(29449):    ... 11 more
12-05 17:14:39.976: E/EmbeddedLogger(1627): App crashed! Process: com.Pakagename
12-05 17:14:39.976: E/EmbeddedLogger(1627): App crashed! Package: com.Pakagename v2 (1.0)

下面是我在oncreate of Subcategory活动中所做的

if(getIntent().hasExtra("categoryid")) {
        if(getIntent().getStringExtra("categoryid") != null) {
            categoryId = Integer.parseInt(getIntent().getStringExtra("categoryid"));
            getSubCategories(categoryId); 
        }
    } else {
        getSubCategories(categoryIdFromPref); // getting an id from pref, that iam saving in onstop
    }

和getSubCategories方法

private void getSubCategories(int categoryId) {
    if(categoryId!=0) {
        query = "SELECT * FROM sub_categories WHERE category_id = "+categoryId+" AND is_archived = 0";
        Log.d("subCategory query", query);
        c = MainActivity.db.rawQuery(query, null);
        c.moveToFirst();
    }
}

logcat在我检查后带我到这条线

c = MainActivity.db.rawQuery(query, null);

我的清单看起来像

    <uses-permission android:name="com.android.vending.BILLING" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.READ_PHONE_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.WAKE_LOCK" />

<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="18" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    <activity
        android:name="com.Pakagename.MainActivity"
        android:label="@string/app_name"
        android:screenOrientation="portrait" >
    </activity>
    <activity
        android:name="com.Pakagename.SplashScreenActivity"
        android:parentActivityName="com.Pakagename.MainActivity"
        android:screenOrientation="portrait"
        android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>

        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:screenOrientation="portrait"
            android:value="com.Pakagename.MainActivity" />
    </activity>
    <activity
        android:name="com.Pakagename.subCategories.SubCategoriesActivity"
        android:screenOrientation="portrait"
    </activity>

如果错误是NullPointerException，则应检查该变量是否不为inicializaste或是否为null

start activity     ComponentInfo{com.Pakagename/com.Pakagename.subCategories.SubCategoriesActivity}:     java.lang.NullPointerException

我希望能帮助你；对不起，我的英语不好。
在
MainActivity
中的静态变量
db
为空。这将导致空指针。如果您解决了这个问题，错误也会得到解决。最好不要在MainActivity中使用静态
db
变量，而是在
SubCategoriesActivity
发布代码和清单文件中重新打开数据库。请查看前面问题的链接，特别是在SubCategoriesActivity中显示方法getSubCategories（）。那里也没有代码。确定，我正在编辑我的问题