DialogHost IsLoaded but not exactly

Bug explanation

I've uploaded a sample project illustrating the issue.

When trying to get a WPF window to open a view using DialogHost on initial startup/show, I noticed that the DialogHost.Show method throws an exception even though the DialogHost instance IsLoaded property was true. I am thinking the DialogHost is loaded but not yet registered in the LoadedInstances HashSet by the time the Window.IsLoaded event triggers.

https://github.com/jbooth88/DialogHostStartupExample

Looks like IsLoaded property is coming from FrameworkElement, so my guess here is that it's not the correct value to identify whether or not the DialogHost is fully initialized. I am not sure of the best way to solve this problem.

Version

5.2.1

Beginne damit, die Startsequenz mit dem verlinkten Projekt DialogHostStartupExample zu reproduzieren, und untersuche DialogHost.Show, FrameworkElement.IsLoaded und den LoadedInstances HashSet. Ermittle, warum Show ausgeführt werden kann, bevor die Registrierung abgeschlossen ist, und überprüfe anschließend den Fix anhand des im Issue beschriebenen Szenarios mit dem Öffnen des initialen Fensters/der initialen Ansicht.