Fix NullReferenceException when hovering over ChangeMarkerMargin in a text file without syntax highlighting.

src/AddIns/DisplayBindings/AvalonEdit.AddIn/Src/DiffControl.xaml.cs

@@ -48,6 +48,7 @@ namespace ICSharpCode.AvalonEdit.AddIn
 		{
 			string language = source.SyntaxHighlighting != null ? source.SyntaxHighlighting.Name : null;
 			editor.TextArea.TextView.LineTransformers.RemoveWhere(x => x is HighlightingColorizer);
+			if (source.SyntaxHighlighting != null)
 				editor.TextArea.TextView.LineTransformers.Insert(0, new CustomizableHighlightingColorizer(source.SyntaxHighlighting.MainRuleSet, CustomizedHighlightingColor.FetchCustomizations(language)));
 			CustomizableHighlightingColorizer.ApplyCustomizationsToDefaultElements(editor, CustomizedHighlightingColor.FetchCustomizations(language));
 			HighlightingOptions.ApplyToRendering(editor, CustomizedHighlightingColor.FetchCustomizations(language));