Fix #352: Folding for Razor files crashes.

When changing a file extension to be a razor file make sure that
any existing folding manager is uninstalled before installing a
Razor folding manager.

src/AddIns/BackendBindings/AspNet.Mvc/Project/Src/Folding/TextEditorWithParseInformationFolding.cs

@@ -42,10 +42,19 @@ namespace ICSharpCode.AspNet.Mvc.Folding
 		{
 			var textEditorAdapter = textEditor as AvalonEditTextEditorAdapter;
 			if (textEditorAdapter != null) {
+				RemoveExistingFoldingManager(textEditorAdapter);
 				foldingManager = FoldingManager.Install(textEditorAdapter.TextEditor.TextArea);
 			}
 		}
 		
+		void RemoveExistingFoldingManager(AvalonEditTextEditorAdapter textEditorAdapter)
+		{
+			var existingFoldingManager = textEditorAdapter.TextEditor.TextArea.GetService(typeof(FoldingManager)) as FoldingManager;
+			if (existingFoldingManager != null) {
+				FoldingManager.Uninstall(existingFoldingManager);
+			}
+		}
+		
 		// TODO disable parse information folding?
 		public bool IsParseInformationFoldingEnabled {
 			get {