fix ChangeMarkerMargin error OnMouseMove when SyntaxHighlighting is null - http://community.sharpdevelop.net/forums/t/12633.aspx

src/AddIns/DisplayBindings/AvalonEdit.AddIn/Src/ChangeMarkerMargin.cs

@@ -168,6 +168,7 @@ namespace ICSharpCode.AvalonEdit.AddIn
 					differ.copyButton.Visibility = Visibility.Collapsed;
 				} else {
 					var baseDocument = new TextDocument(changeWatcher.BaseDocument.Text);
+					if (differ.editor.SyntaxHighlighting != null) {
 						var mainHighlighter = new DocumentHighlighter(baseDocument, differ.editor.SyntaxHighlighting.MainRuleSet);
 						var popupHighlighter = differ.editor.TextArea.GetService(typeof(IHighlighter)) as DocumentHighlighter;
 						
@@ -176,6 +177,7 @@ namespace ICSharpCode.AvalonEdit.AddIn
 						else
 							popupHighlighter.InitialSpanStack = mainHighlighter.GetSpanStack(lineInfo.OldStartLineNumber);
 					}
+				}
 				
 				differ.revertButton.Click += delegate {
 					if (hasNewVersion) {