Unable to add window -- token null is not for an application---各种bug

10-18 18:49:02.767: E/InputEventSender(6274): Exception dispatching finished signal.
10-18 18:49:02.767: E/MessageQueue-JNI(6274): Exception in MessageQueue callback: handleReceiveCallback
10-18 18:49:02.774: E/MessageQueue-JNI(6274): android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
10-18 18:49:02.774: E/MessageQueue-JNI(6274): at android.view.ViewRootImpl.setView(ViewRootImpl.java:754)
10-18 18:49:02.774: E/MessageQueue-JNI(6274): at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:267)
10-18 18:49:02.774: E/MessageQueue-JNI(6274): at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:70)
10-18 18:49:02.774: E/MessageQueue-JNI(6274): at android.app.Dialog.show(Dialog.java:347)

10-18 18:49:02.774: E/MessageQueue-JNI(6274):at android.app.AlertDialog.show(AlertDialog.java:1367)

错误原因：又用的是  context    只能用activity.this

AlertDialog.Builder builder = new Builder(MainActivity.this);