Android中的Firebase聊天限制错误？

Question

有些时间并没有错误，它的运行没有错误但是大部分时间都在这段代码中出错了，我为此尝试了很多东西。我尝试在onStart()上调用此代码，并在onActivityCreted()上尝试此操作。但我得到同样的错误。这是我之前聊天的编码。我在limit()中收到了错误。

final ListView listView = getListView();
    mChatListAdapter = new FirebaseListAdapter<>(mFirebaseRef.limit(50), Chat.class, R.layout.chat_item, R.layout.chat_item_other,
            R.layout.third_layout_chat, R.layout.fourth_layout_chat, getActivity(), chat_id);
    listView.setAdapter(mChatListAdapter);
    mChatListAdapter.registerDataSetObserver(new DataSetObserver() {
        @Override
        public void onChanged() {
            super.onChanged();
            listView.setSelection(mChatListAdapter.getCount() - 1);
        }
    });

这是错误。

FATAL EXCEPTION: main Process: com.socialapp.android, PID: 8112 Theme: themes:{}                                                                   java.lang.RuntimeException: Unable to start activity ComponentInfo{com.socialapp.android/com.socialapp.chat.ChatViewActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.firebase.client.Query com.firebase.client.Firebase.limit(int)' on a null object reference
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2450)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2520)
   at android.app.ActivityThread.-wrap11(ActivityThread.java)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363)
   at android.os.Handler.dispatchMessage(Handler.java:102)
   at android.os.Looper.loop(Looper.java:148)
   at android.app.ActivityThread.main(ActivityThread.java:5466)
   at java.lang.reflect.Method.invoke(Native Method)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.firebase.client.Query com.firebase.client.Firebase.limit(int)' on a null object reference
   at com.socialapp.chat.ChatViewFragment.onStart(ChatViewFragment.java:609)
   at android.app.Fragment.performStart(Fragment.java:2244)
   at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1002)
   at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1148)
   at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1130)
   at android.app.FragmentManagerImpl.dispatchStart(FragmentManager.java:1958)
   at android.app.FragmentController.dispatchStart(FragmentController.java:163)
   at android.app.Activity.performStart(Activity.java:6274)
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2413)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2520) 
   at android.app.ActivityThread.-wrap11(ActivityThread.java) 
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363) 
   at android.os.Handler.dispatchMessage(Handler.java:102) 
   at android.os.Looper.loop(Looper.java:148) 
   at android.app.ActivityThread.main(ActivityThread.java:5466) 
   at java.lang.reflect.Method.invoke(Native Method) 
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)