Fix null reference exception on disposing EDM View.

When an exception is thrown when loading a .edmx file
the EDMDesignerViewContent's Dispose method was not
handling the EDMView being null and causing the
underlying exception to be hidden.

src/AddIns/DisplayBindings/Data/ICSharpCode.Data.EDMDesigner.Core.UI/DisplayBinding/EDMDesignerViewContent.cs

@@ -174,8 +174,10 @@ namespace ICSharpCode.Data.EDMDesigner.Core.UI.DisplayBinding
 
 		public override void Dispose()
 		{
-			if (CSDLDatabaseTreeViewAdditionalNode.Instance.CSDLViews.Contains(_edmView.CSDL))
+			if (_edmView != null) {
-				CSDLDatabaseTreeViewAdditionalNode.Instance.CSDLViews.Remove(_edmView.CSDL);
+				if (CSDLDatabaseTreeViewAdditionalNode.Instance.CSDLViews.Contains(_edmView.CSDL))
+					CSDLDatabaseTreeViewAdditionalNode.Instance.CSDLViews.Remove(_edmView.CSDL);
+			}
 
 			EDMDesignerChangeWatcher.RemoveEDMDesignerViewContent(this);
 		}