startSupportActionMode（）throw NullPointerException

Question

AppCompatActivityの中にstartSupportActionMode()と呼んでいます。このアクティビティは、アンドロイドスタジオの「基本アクティビティ」をクリックすることで作成されます。つまり、AppBarLyoutとToolBarを使用してActionBarを作成します。ここに私の活動

import android.support.v7.view.ActionMode; 
public class ListActivity extends AppCompatActivity implements AdapterView.OnItemClickListener,AdapterView.OnItemLongClickListener, ActionMode.Callback{ 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_list); 
    Toolbar toolbar = findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 

    FloatingActionButton fab = findViewById(R.id.fab); 
    fab.setOnClickListener(view -> { 
     Intent intent = new Intent(this, MainActivity.class); 
     startActivity(intent); 
    }); 

    Button buttonLoadResult = findViewById(R.id.buttonLoadResult); 
    buttonLoadResult.setOnClickListener(v -> { 
     Intent intent = new Intent(ListActivity.this, MainActivity.class); 
     startActivity(intent); 
    }); 

    emptyStateLayout = findViewById(R.id.layoutEmptyState); 
    listAdapter = new ListAdapter(this, getFileList()); 
    listView = findViewById(R.id.listViewResultList); 
    listView.setOnItemClickListener(this); 
    listView.setOnItemLongClickListener(this); 
    listView.setAdapter(listAdapter); 
    listView.setLongClickable(true); 
    listView.setChoiceMode(AbsListView.CHOICE_MODE_MULTIPLE_MODAL); 

    setAppropriateViews(); 
} 

//Some codes... 

@Override 
public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) { 
    listView.setItemChecked(position,true); 
    if (activeMode == null) { 
     activeMode = this.startSupportActionMode(this); 
     return true; 
    } 
    return false; 
} 
@Override 
public boolean onCreateActionMode(ActionMode mode, Menu menu) { 
    getMenuInflater().inflate(R.menu.menu_action_mode, menu); 
    return true; 
} 

@Override 
public boolean onPrepareActionMode(ActionMode mode, Menu menu) { 
    return false; 
} 

@Override 
public boolean onActionItemClicked(ActionMode mode, MenuItem item) { 
    return false; 
} 

@Override 
public void onDestroyActionMode(ActionMode mode) { 
    activeMode = null; 
} 
} 

それは次のログ

に与えている

java.lang.NullPointerException: Attempt to invoke interface method 'boolean android.view.ActionMode$Callback.onCreateActionMode(android.view.ActionMode, android.view.Menu)' on a null object reference 
                       at com.android.internal.policy.DecorView$ActionModeCallback2Wrapper.onCreateActionMode(DecorView.java:2286) 
                       at android.support.v7.view.SupportActionModeWrapper$CallbackWrapper.onCreateActionMode(SupportActionModeWrapper.java:157) 
                       at android.support.v7.app.AppCompatDelegateImplV9$ActionModeCallbackWrapperV9.onCreateActionMode(AppCompatDelegateImplV9.java:1793) 
                       at android.support.v7.app.AppCompatDelegateImplV9.startSupportActionModeFromWindow(AppCompatDelegateImplV9.java:824) 
                       at android.support.v7.app.AppCompatDelegateImplV9.startSupportActionMode(AppCompatDelegateImplV9.java:707) 
                       at android.support.v7.app.AppCompatDelegateImplV14$AppCompatWindowCallbackV14.startAsSupportActionMode(AppCompatDelegateImplV14.java:292) 
                       at android.support.v7.app.AppCompatDelegateImplV23$AppCompatWindowCallbackV23.onWindowStartingActionMode(AppCompatDelegateImplV23.java:66) 
                       at android.support.v7.view.WindowCallbackWrapper.onWindowStartingActionMode(WindowCallbackWrapper.java:160) 
                       at com.android.internal.policy.DecorView.startActionMode(DecorView.java:835) 
                       at com.android.internal.policy.DecorView.startActionModeForChild(DecorView.java:816) 
                       at android.view.ViewGroup.startActionModeForChild(ViewGroup.java:828) 
                       at android.view.ViewGroup.startActionModeForChild(ViewGroup.java:828) 
                       at android.view.ViewGroup.startActionModeForChild(ViewGroup.java:828) 
                       at android.view.ViewGroup.startActionModeForChild(ViewGroup.java:828) 
                       at android.view.ViewGroup.startActionModeForChild(ViewGroup.java:828) 
                       at android.view.ViewGroup.startActionModeForChild(ViewGroup.java:828) 
                       at android.view.View.startActionMode(View.java:5828) 
                       at android.view.View.startActionMode(View.java:5812) 
                       at android.widget.AbsListView.performLongPress(AbsListView.java:3222) 
                       at android.widget.AbsListView$CheckForLongPress.run(AbsListView.java:3148) 
                       at android.os.Handler.handleCallback(Handler.java:751) 
                       at android.os.Handler.dispatchMessage(Handler.java:95) 
                       at android.os.Looper.loop(Looper.java:154) 
                       at android.app.ActivityThread.main(ActivityThread.java:6119) 
                       at java.lang.reflect.Method.invoke(Native Method) 
                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

Answer 1

私の間違いを発見し、私はそれを変更する場合には、以下の行

listView.setChoiceMode(AbsListView.CHOICE_MODE_MULTIPLE_MODAL); 

によるものであったし、それが働いているました〜

listView.setChoiceMode(AbsListView.CHOICE_MODE_MULTIPLE); 

しかし、私は私はまだそれがなぜNullPointerExceptionを投げたのか、なぜCHOICE_MODE_MULTIPLE_MODALがonCreateActionMode()になるのかを混乱させる。