当我想启动程序中的Progress Dialog 时发生的异常：
android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
at android.view.ViewRoot.setView(ViewRoot.java:531)
at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:177)
at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:91)
at android.app.Dialog.show(Dialog.java:241)
at android.app.ProgressDialog.show(ProgressDialog.java:107)
at android.app.ProgressDialog.show(ProgressDialog.java:90)
请问这是什么原因?
