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

new AlertDialog.Builder(getApplicationContext())

.setIcon(android.R.drawable.ic_dialog_alert)

.setTitle("提示")

.setMessage("上下文报错")

.setPositiveButton("Yes", positiveListener).setNegativeButton(

"No", negativeListener).create().show();

错误原因：AlertDialog.Builder(getApplicationContext())
不能用应用程序的上下文，应该用当前Activity的Context，因为只有一个Activity才能添加一个Dialog
正确应该：

new AlertDialog.Builder(MainActivity.this)

.setIcon(android.R.drawable.ic_dialog_alert)

.setTitle("提示")

.setMessage("一个Activity才能添加一个Dialog")

.setPositiveButton("Yes", positiveListener).setNegativeButton(

"No", negativeListener).create().show();