Android Crash - org.cocos2dx.lib.Cocos2dxGLSurfaceView.queueEvent(java.lang.Runnable)' on a null object reference

cocos2d-x
1

java.lang.NullPointerException
Attempt to invoke virtual method ‘void org.cocos2dx.lib.Cocos2dxGLSurfaceView.queueEvent(java.lang.Runnable)’ on a null object reference
org.cocos2dx.lib.Cocos2dxActivity.runOnGLThread(Cocos2dxActivity.java:223)

I show AD from Facbook or Admob SDK in UIThread,when the AD closed,i call the result to GLThread, my code as follow:
--------------------------------code start------------------------------------------------------
public class AppActivity extends Cocos2dxActivity {
protected static AppActivity instance = null;

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
instance = this;
}

//----------------------------------advertisingBackRunnable(UI Thread —>GL Thread)-----------------------------------------
static class advertisingBackRunnable implements Runnable {
private boolean success = false;
private String placeID = “”;
private String adName = “”;
private String placeBack = “”;

    public advertisingBackRunnable(boolean _success, String _placeID, String _adName, String _placeBack) {
        success = _success;
        placeID = _placeID;
        adName = _adName;
        placeBack = _placeBack;
    }

    @Override
    public void run() {
        String strSucess = "0";
        if (success == true)
            strSucess = "1";
        Log.i("cocos2d-x", String.format("advertisingBack(%s,%s,%s,%s)", strSucess, placeID, adName, placeBack));
        //public native static void advertisingBack(boolean success,String placeID,String adName,String placeBack);
        advertisingBack(success, placeID, adName, placeBack);
    }
}

public native static void advertisingBack(boolean success, String placeID, String adName, String placeBack);
}
----------------------------------end--------------------------------------------------------------------
when the AD finish, i call like this:

advertisingStartRunnable run = new advertisingStartRunnable(false, m_strADPlace, “null”, m_strADBack);
instance.runOnGLThread(run);

It crash sometime and show error like this:
java.lang.NullPointerException
Attempt to invoke virtual method ‘void org.cocos2dx.lib.Cocos2dxGLSurfaceView.queueEvent(java.lang.Runnable)’ on a null object reference
org.cocos2dx.lib.Cocos2dxActivity.runOnGLThread(Cocos2dxActivity.java:223)

Anyone has the same problems ? Or something wrong in my code?