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

错误产生：

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

private Context mcontext;

@Override
protected void onCreate(Bundle savedInstanceState) {mcontext = getApplicationContext();
System.out.println("mcontext=" + mcontext);

}

private Context mcontext;
@Override
protected 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();