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

还没有人认领这个 Issue。

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)

从 MaterialDesignThemes.Wpf/DialogHost.cs 的第 289–300 行附近开始，重点查看 IsOpenPropertyChangedCallback 中的延迟 continuation。重现一个快速打开并关闭的序列，或快速导航离开的序列，并跟踪哪个引用可能会在延迟期间变得不可用。完成标准是该序列不再产生报告中的 NullReferenceException 或未观察到的任务异常。

由索引模型根据 Issue 内容生成。