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Using the DisplayAlert method on pages using the NavigationPage as a wrapper results in a "System.InvalidOperation" exception when the "cancel" button is selected.
Steps to reproduce
1) In the attached project, select the "reset password" button on the login page.
2) On the reset password page, select the "reset password" button without entering any information into the username text field.
3) When the alert appears saying that the username is required, select the dismiss button on the alert.
The user is brought back to the reset password page to enter their username to reset their password.
The application crashes with the following error:
System.InvalidOperationException was unhandled
Message: An unhandled exception of type 'System.InvalidOperationException' occurred in System.Windows.ni.dll
Additional information: An attempt was made to transition a task to a final state when it had already completed.
The code works for the reported user in iOS and Android. The exception also does not occur when using Display alert on the login page.
Microsoft Visual Studio 2013
Version 12.0.30723.00 Update 3
Xamarin Forms 1.2.26243
Possibly related bug: https://bugzilla.xamarin.com/show_bug.cgi?id=23358
Fixed for next update.