Java 在导航抽屉布局中为片段使用布局文件

我有一个android应用程序代码，它使用导航抽屉布局，以片段为主要内容。 到目前为止，我已经为抽屉中的所有片段项目创建了一个特定的片段类。现在我有一个不同的布局，我想得到作为一个项目在我的抽屉以及。布局是一个LiniarLayout，在xml characteristic.xml中定义

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="10dp">
    <LinearLayout android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp">
        <TextView android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/label_device_address"
            android:textSize="18sp"/>
        <Space android:layout_width="5dp"
            android:layout_height="wrap_content"/>
        <TextView android:id="@+id/device_address"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textSize="18sp"/>
     </LinearLayout>
     <LinearLayout android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp">
        <TextView android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/label_state"
            android:textSize="18sp"/>
        <Space android:layout_width="5dp"
            android:layout_height="wrap_content"/>
        <TextView android:id="@+id/connection_state"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/disconnected"
            android:textSize="18sp"/>
    </LinearLayout>
    <LinearLayout android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp">
        <TextView android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/label_data"
            android:textSize="18sp"/>
        <Space android:layout_width="5dp"
            android:layout_height="wrap_content"/>
        <TextView android:id="@+id/data_value"
           android:layout_width="match_parent"
           android:layout_height="wrap_content"
           android:text="@string/no_data"
           android:textSize="18sp"/>
    </LinearLayout>
   <ExpandableListView android:id="@+id/gatt_services_list"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"/>
</LinearLayout>

我希望能够使用它在片段之间切换（除了新的片段3外，它工作得很好）：

但是，它似乎不起作用： mGattServicesList.setOnChildClickListener（ServicesListClickListener）； 失败于 E/AndroidRuntime（30644）：由以下原因引起：java.lang.NullPointerException:尝试调用空对象引用上的虚拟方法“void android.widget.ExpandableListView.setOnChildClickListener（android.widget.ExpandableListView$OnChildClickListener）”

---

有人能告诉我我做错了什么吗

mGattServicesList

是否在

activity\u main

layout中定义？mGattServicesList是我的主活动类中的一个变量，我试图将它从fragment.xml文件分配给ExpandableListView

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //create adapters early
        mAdapter = new DeviceAdapter(this, false);

        mFilterAdapter = new DeviceAdapter(this, true);
        setContentView(R.layout.activity_main);
        mFragmentManager = getFragmentManager();
        mNavigationDrawerFragment = (NavigationDrawerFragment)
                mFragmentManager.findFragmentById(R.id.navigation_drawer);
        mTitle = getTitle();

        // Set up the drawer.
        mNavigationDrawerFragment.setUp(
                R.id.navigation_drawer,
                (DrawerLayout) findViewById(R.id.drawer_layout));

        mDevices = new HashMap<String, ScanResultContent.BtLeDeviceItem>();
        mFilteredDevices = new HashMap<String, ScanResultContent.BtLeDeviceItem>();
        //setup fragment 1
        mScanResultFragment = new ResultListFragment();
        //setup fragment 2
        mFilteredScanResultFragment = new ResultListFragment();
        mScanResultFragment.setListAdapter(mAdapter);
        //setup fragment 3
        mDeviceInfoFragment = DeviceInfoFragment.newInstance("");

        // Sets up UI references for the Device Info viewer.
        mConnectionState = (TextView) findViewById(R.id.connection_state);
        mDataField = (TextView) findViewById(R.id.data_value);
        mGattServicesList = (ExpandableListView) findViewById(R.id.gatt_services_list);
        mGattServicesList.setOnChildClickListener(servicesListClickListner);

        mFilteredScanResultFragment.setListAdapter(mFilterAdapter);
        // setup default fragment shown
        mFragmentManager.beginTransaction().add(R.id.container, mScanResultFragment).commit();
    }

public class DeviceInfoFragment extends Fragment {

    public DeviceInfoFragment(){
        super();
    }

    public static DeviceInfoFragment newInstance(String text) {

        DeviceInfoFragment f = new DeviceInfoFragment();

        Bundle b = new Bundle();
        b.putString("text", text);
        f.setArguments(b);
        return f;
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v =  inflater.inflate(R.layout.gatt_characteristics_layout, container, false);    
        return v;
    }
}

@Override
public void onNavigationDrawerItemSelected(int position) {
    // update the main content by replacing fragments
    Fragment f = null;
    switch (position+1){
        case 1:
            f = mScanResultFragment;
            mTitle = getString(R.string.title_devicescan);
            break;
        case 2:
            f = mFilteredScanResultFragment;
            mTitle = getString(R.string.title_results);
            break;
        case 3:
            f = mDeviceInfoFragment;
            mTitle = getString(R.string.title_device_info);
            break;
    }
    if(f != null) {
        mFragmentManager.beginTransaction()
                .replace(R.id.container, f)
                .commit();
    }
    restoreActionBar();
}