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In order to display the notification that an exception has occurred you don't even need a handler for the ExceptionOccurred event. You just need to initialize the ApplicationUsageHelper class and an instance of the Diagnostics component. I have attached a sample application to show you the expected behavior. In this application, an exception is thrown when the MainPage is loaded to illustrate the message displayed by RadDiagnostics.
Please note that if you start the application without debugging you will immediately see the message and if you start debugging, VisualStudio will notify you first that an exception has occurred and if you continue debugging (F5), you will eventually see the message in the application as well.
I hope this helps. Let me know if you still don't see the message of the exception.
All the best,
the Telerik team
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