Everybody that is doing XAML editing has hit this issue one moment in time – the editor is buggy. First of all – the designer is really useless. Second, if you keep editors open side-by-side you will eventually get a Visual Studio crash. Also from time to time you won’t be able to open the XAML file because apparently it is open but hidden. Finally, I would mention that the XAML editor is a lot slower than the other Visual Studio editors.
For all the issues above, there is a very simple solution which will make your life easier. You just need to right click an XAML file and choose Open With…. In the dialog select Source Code (Text) Editor and make it the default editor. You will get exactly the same features as in the XAML editor (Intellisense etc.), but without the crashes and performance issues.