DialogHost IsLoaded but not exactly

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### 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

Inizia riproducendo la sequenza di avvio con il progetto collegato DialogHostStartupExample e ispeziona DialogHost.Show, FrameworkElement.IsLoaded e l’HashSet LoadedInstances. Determina perché Show può essere eseguito prima che la registrazione sia completata, quindi verifica il fix con lo scenario di apertura della finestra/visualizzazione iniziale descritto nell’issue.

Scritto dal modello di indicizzazione a partire dal testo della issue.