Android android.view.WindowManager$BadTokenException: Unable to add window

Question

I am linking my android app with facebook and call an Async task to authenticate fb user and then in handler calling another Async task to link the fb account in my database. On recieving info from second Async task I am showing process dialog with context 'this'. There are process dialog in both Async tasks also. Sometimes it works fine but sometimes it show

android.view.WindowManager$BadTokenException: Unable to add window -- token android.os.BinderProxy@422608e8 is not valid; is your activity running?