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Navigation from CustomMessageBox Dismissed event causes NullReferenceException
Steps to reproduce:
1) Create a CustomMessageBox object.
2) Add a Dismissed event handler to this object.
3) In the Dismissed event handler, add a Navigation to another page.
4) Run the app and dismiss the CustomMessageBox. The app should throw a NullReferenceException.
What happens (from what I can understand) is the CustomMessageBox starts dismissing, runs the dismiss event as expected, then it tries to navigate, also as expected. It then gets dismissed a second time, at the end of which (the second Dismissed event), a NullReferenceException is thrown. I believe there is some cleanup that is happening twice in this case. I will do some research in the source today and see if I can't isolate it.
For anyone else looking at this, possible fixes may include:
A try/catch in the cleanup function.
Checking for null before accessing the cleanup.
Preventing the dismiss from being called if previously dismissed, possibly by checking for navigation events.
Adding a post-Dismissed event with which to Navigate from.