Exception occurred and I can't close the window.

Dear,
The problem takes place when I frequently click on the Toolbar button, which
does a postback, and sometimes the alert msg shows: (for slow computers, it comes easier)

  Exception Occurred in initialization!
  Error Message:undifined, e.id:undefined
And then I can not close the window, but it's still working.
Is something wrong when the page is loaded?
 
Thanks for any suggestions,
Albert.

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