启动程序中的Progress Dialog 时的错误

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?

context用activity.this