在托管活动中的onResume之后，onMessageReceived回调中的fragment为null

Question

当活动恢复并且我从接收器应用程序收到消息时，对我的片段的引用为null。在活动的原始启动时，收到回调的消息可以在我的片段中调用一个方法而没有任何问题。但是，一旦应用程序关闭，恢复并重新附加消息回调，如果我尝试在onResume中保留对片段的引用，则片段似乎返回null或者我得到“片段未附加到活动”错误。令我感到困惑的是，片段在onResume函数中不为null，但它在messageReceived回调中...

这是我的代码：

class DashboardCastMainChannel implements Cast.MessageReceivedCallback {
    public String getNamespace() {
        return "urn:x-cast:myapp";
    }
    @Override
    public void onMessageReceived(CastDevice castDevice, String namespace, String message) {
        //Returns null only after activity resume
        DashboardCastFragment fragment = (DashboardCastFragment) getFragmentManager().findFragmentById(R.id.dashboard_main_fragment);
        JSONObject messageJSON;
        try {
            messageJSON= new JSONObject(message);
            fragment.handleMediaMessage(messageJSON); //Crashes because fragment is null after resume
        } catch (JSONException e) {
            e.printStackTrace();
        }
    }

}

@Override
protected void onResume () {
    super.onResume();

    //Fragment exists and not null
    mainFragment = (DashboardCastFragment) getFragmentManager().findFragmentById(R.id.dashboard_main_fragment); //Fragment exists and not null

    int playServicesAvailable = GooglePlayServicesUtil.isGooglePlayServicesAvailable(getApplicationContext()); 
    if (playServicesAvailable == ConnectionResult.SUCCESS) {
        mediaRouter.addCallback(mediaRouteSelector, mediaRouterCallback, MediaRouter.CALLBACK_FLAG_PERFORM_ACTIVE_SCAN);

        //Attach message listener back to activity
        if (castChannel != null) {
            try {
                Cast.CastApi.setMessageReceivedCallbacks(
                        apiClient,
                        castChannel.getNamespace(),
                        castChannel);
            } catch (IOException e) {
                Log.e(TAG, "Exception while creating channel", e);
            } catch (IllegalStateException e) {
                Log.e(TAG, "Failed to setup media channel", e);
            }
        }
    } else {
        GooglePlayServicesUtil.getErrorDialog(playServicesAvailable, this, 1).show();
    }

}

@Override
protected void onPause () {
    mediaRouter.removeCallback(mediaRouterCallback);

    if (castChannel != null) {
        try {
            Cast.CastApi.removeMessageReceivedCallbacks(
                    apiClient,
                    castChannel.getNamespace());
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    super.onPause();
}

这是我的错误日志：

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.rir.dashboardcast.DashboardCastFragment.handleMediaMessage(org.json.JSONObject)' on a null object reference
    at com.rir.dashboardcast.DashboardCastActivity$DashboardCastMainChannel.onMessageReceived(DashboardCastActivity.java:582)
    at com.google.android.gms.internal.ij$c$4.run(Unknown Source)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5221)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

提前谢谢