DialogHost can throw exception when opening and closing dialog in quick succession

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DialogHost can throw exception when opening and closing dialog in quick succession.

## Background

I have a view which is password protected. The password dialog is shown when the user opens the view. The dialog is using a `DialogHost` placed on the view and is the only "client" of the dialog host. If the user navigates away from the view, the dialog will be closed.

## Problem

If the dialog is shown and the user navigates away from the view shortly after there is a chance that the `DialogHost` will throw a `NullReferenceException`.

```
2018-10-04 09:02:10.870 +02:00 [Error ] [02] Unhandled exception occurred. Origin=System.Threading.Tasks.TaskScheduler.UnobservedTaskException,
System.AggregateException: A Task's exception(s) were not observed either by Waiting on the Task or accessing its Exception property. As a result, the unobserved exception was rethrown by the finalizer thread. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at MaterialDesignThemes.Wpf.DialogHost.<>c__DisplayClass38_1.b__2(Task t) in C:\projects\materialdesigninxamltoolkit\MaterialDesignThemes.Wpf\DialogHost.cs:line 296
at System.Threading.Tasks.ContinuationResultTaskFromTask`1.InnerInvoke()
at System.Threading.Tasks.Task.Execute()
--- End of inner exception stack trace ---
---> (Inner Exception #0) System.NullReferenceException: Object reference not set to an instance of an object.
at MaterialDesignThemes.Wpf.DialogHost.<>c__DisplayClass38_1.b__2(Task t) in C:\projects\materialdesigninxamltoolkit\MaterialDesignThemes.Wpf\DialogHost.cs:line 296
at System.Threading.Tasks.ContinuationResultTaskFromTask`1.InnerInvoke()
at System.Threading.Tasks.Task.Execute()<---
```

My guess is that since there is a delay when opening a dialog it is possible that some reference become `null` during the delay. I think that the `child` variable should not change value but it could be possible that its `Dispatcher` property becomes `null` when the object is removed from visual tree or something like that.

Anyway, the code might need some more protective `null` checking.

```
dialogHost.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() =>
{
CommandManager.InvalidateRequerySuggested();
UIElement child = dialogHost.FocusPopup();

if (child != null)
{
//https://github.com/ButchersBoy/MaterialDesignInXamlToolkit/issues/187
//totally not happy about this, but on immediate validation we can get some weird looking stuff...give WPF a kick to refresh...
Task.Delay(300).ContinueWith(t => child.Dispatcher.BeginInvoke(new Action(() => child.InvalidateVisual())));
}
}));
```

see [DialogHost.cs](https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/blob/master/MaterialDesignThemes.Wpf/DialogHost.cs#L289-L300)

Inizia in MaterialDesignThemes.Wpf/DialogHost.cs intorno alle righe 289–300, soprattutto nella continuazione ritardata di IsOpenPropertyChangedCallback. Riproduci una sequenza rapida di apertura e chiusura oppure di navigazione verso un’altra pagina e traccia quale riferimento può diventare non disponibile durante il ritardo. Il lavoro è completato quando la sequenza non produce più la NullReferenceException segnalata né un’eccezione di task non osservata.

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