【Android】Attempt to invoke virtual method 'com.android.volley.Request com.android.volley.RequestQueue.add(com.android.volley.Request)' on a null object reference

當使用Volley時

不必為每一次HTTP請求都創建一個RequestQueue對象，推薦在application中作初始化

所以

public class MyApplication extends Application {

    public static RequestQueue requestQueue;

    @Override
    public void onCreate() {
        super.onCreate();
        requestQueue = Volley.newRequestQueue(this);
    }
}

之後要呼叫時

        RequestQueue mQueue = MyApplication.requestQueue;
        StringRequest getStringRequest = new StringRequest("網址", new ResponseListener(), 

        new ResponseErrorListener());
        mQueue.add(getStringRequest);

但執行結果出現
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.android.volley.Request com.android.volley.RequestQueue.add(com.android.volley.Request)' on a null object reference

解決辦法
ref:
 http://stackoverflow.com/a/31302226

AndroidManifest.xml中

<application android:name="YOURPACKAGENAME.AppController" 
             android:allowbackup="true" 
             android:icon="@drawable/ic_launcher" 
             android:label="@string/app_name"
             android:theme="@style/AppTheme">