我已经实例化了一个扩展 CameraFragment（并实现 TabListener）的片段类型，并且我试图从父 Activity 调用 autoFocus()。从扩展 CameraFragment 的片段调用 autoFocus() 工作正常，但从父 Activity 调用它会导致 NPE。我正在主 Activity 中监听 onKeyDown() 事件，该事件按照 logcat 的预期工作。这是我的 NPE 跟踪：
09-10 10:00:16.410: D/app(5283): onKeyDown: 80
09-10 10:00:16.415: D/AndroidRuntime(5283): Shutting down VM
09-10 10:00:16.415: W/dalvikvm(5283): threadid=1: thread exiting with uncaught exception (group=0x40f7b2a0)
09-10 10:00:16.425: E/AndroidRuntime(5283): FATAL EXCEPTION: main
09-10 10:00:16.425: E/AndroidRuntime(5283): java.lang.NullPointerException
09-10 10:00:16.425: E/AndroidRuntime(5283): at com.commonsware.cwac.camera.CameraFragment.autoFocus(CameraFragment.java:96)
09-10 10:00:16.425: E/AndroidRuntime(5283): at com.me.app.appCameraFragment.callAutoFocus(cameraFragment.java:232)
09-10 10:00:16.425: E/AndroidRuntime(5283): at com.me.app.MainTabActivity.onKeyDown(MainTabActivity.java:264)
09-10 10:00:16.425: E/AndroidRuntime(5283): at android.view.KeyEvent.dispatch(KeyEvent.java:2705)
09-10 10:00:16.425: E/AndroidRuntime(5283): at android.app.Activity.dispatchKeyEvent(Activity.java:2423)
09-10 10:00:16.425: E/AndroidRuntime(5283): at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2019)
09-10 10:00:16.425: E/AndroidRuntime(5283): at android.view.ViewRootImpl.deliverKeyEventPostIme(ViewRootImpl.java:3851)
09-10 10:00:16.425: E/AndroidRuntime(5283): at android.view.ViewRootImpl.handleImeFinishedEvent(ViewRootImpl.java:3799)
09-10 10:00:16.425: E/AndroidRuntime(5283): at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:2934)
09-10 10:00:16.425: E/AndroidRuntime(5283): at android.os.Handler.dispatchMessage(Handler.java:99)
09-10 10:00:16.425: E/AndroidRuntime(5283): at android.os.Looper.loop(Looper.java:137)
09-10 10:00:16.425: E/AndroidRuntime(5283): at android.app.ActivityThread.main(ActivityThread.java:4921)
09-10 10:00:16.425: E/AndroidRuntime(5283): at java.lang.reflect.Method.invokeNative(Native Method)
09-10 10:00:16.425: E/AndroidRuntime(5283): at java.lang.reflect.Method.invoke(Method.java:511)
09-10 10:00:16.425: E/AndroidRuntime(5283): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1036)
09-10 10:00:16.425: E/AndroidRuntime(5283): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:803)
09-10 10:00:16.425: E/AndroidRuntime(5283): at dalvik.system.NativeStart.main(Native Method)