Added null checks for managedCefBrowserAdapter.

Got a nullref in ChromiumWebBrowser.Load(string url) if bound address changes after dispose was called. Therefor added null checks for managedCefBrowserAdapter.
For this `PR` I'd prefer not to introduce null checks to public facing methods. As it stands we've got null checks for calls that are called by the `WPF` framework in general, not in user called code.