Make sure the XamlTextViewCreationListerner's RDT watcher doesn't crash

A few places we might try to remove something that we shouldn't.

src/VisualStudio/Xaml/Impl/Implementation/XamlTextViewCreationListener.cs

@@ -167,9 +167,12 @@ private void OnDocumentMonikerChanged(IVsHierarchy hierarchy, string oldMoniker,
         private void OnDocumentClosed(uint docCookie)
         {
             RunningDocumentInfo info = _rdt.Value.GetDocumentInfo(docCookie);
-            if (_xamlProjects.TryGetValue(info.Hierarchy, out VisualStudioProject project))
+            if (info.Hierarchy != null && _xamlProjects.TryGetValue(info.Hierarchy, out VisualStudioProject project))
             {
-                project.RemoveSourceFile(info.Moniker);
+                if (project.ContainsSourceFile(info.Moniker))
+                {
+                    project.RemoveSourceFile(info.Moniker);
+                }
             }
         }
     }