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Created attachment 10694 [details]
The FormsApplicationDelegate class's FinishedLaunching method calls SetMainPage to set window's RootViewController and make the window key and visible. UpdatePage (called when the MainPage property of the application changes) also sets the window's root view controller, but it does an additional step of disposing the old root view controller.
As a result, ff you don't set the MainPage until after FinishedLaunching then UpdatePage crashes with a NullReferenceException when trying to dispose the old (null) root view controller.
SetMainPage should just call UpdateMainPage so that these two functions do the same thing, and then UpdateMainPage should check for null before trying to dispose the old view controller.
To reproduce run the attached project.
FYI, I know that it's expected to set the MainPage in the app's constructor, but I can't think of a technical reason that setting it in OnStart instead shouldn't work as expected.
I am also getting the same error using Xamarin Forms 18.104.22.16855. Thank you.
Should be fixed in 1.4.3 final
I have checked this issue with xamarin.forms.22.214.171.12464-pre3 and its working fine at my end.
Hence closing this issue.
Xamarin Studio 5.9.3 (build 1)
Installation UUID: 1a096c6f-0678-402e-89b2-a2c10f7e80e4
Mono 4.0.1 ((detached/ed1d3ec)
GTK+ 2.24.23 (Raleigh theme)
Package version: 400010044
Xcode 6.2 (6776)
Xamarin.iOS : 126.96.36.199 (Enterprise Edition)
Xamarin.Android : 188.8.131.52 (Enterprise Edition)
Xamarin.Mac : 184.108.40.206 (Enterprise Edition)
Mac OS X 10.9.4