android.view.WindowManager$BadTokenException: Unable to add window — token null

错误产生：

private Context mcontext;@Overrideprotected void onCreate(Bundle savedInstanceState) {mcontext = getApplicationContext();System.out.println("mcontext=" + mcontext);}

1. private Context mcontext;   
2.   
3.     @Override  
4.     protected void onCreate(Bundle savedInstanceState) {mcontext = getApplicationContext();   
5.         System.out.println("mcontext=" + mcontext);   
6.   
7.     }  

private Context mcontext;@Overrideprotected void onCreate(Bundle savedInstanceState) {mcontext = getApplicationContext();System.out.println("mcontext=" + mcontext);}

导致报这个错是在于new AlertDialog.Builder(mcontext)，虽然这里的参数是AlertDialog.Builder(Context context)但我们不能使用getApplicationContext()获得的Context,而必须使用Activity,因为只有一个Activity才能添加一个窗体。 

new AlertDialog.Builder(MyActivity.this).setIcon(android.R.drawable.ic_dialog_alert).setTitle("Warnning").setMessage("You forget to write the message. Do you want to fill out it ??").setPositiveButton("Yes", positiveListener).setNegativeButton("No", negativeListener).create().show();