Fix SD-1760 - NullReferenceException when double-clicking button in WPF designer when .xaml file does not belong to a project

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.AddIn/Src/AbstractEventHandlerService.cs

@@ -26,13 +26,21 @@ namespace ICSharpCode.WpfDesign.AddIn
 		
 		protected IProjectContent GetProjectContent()
 		{
-			IProject p = ProjectService.OpenSolution.FindProjectContainingFile(viewContent.PrimaryFileName);
+			IProject p = FindProjectContainingFile();
 			if (p != null)
 				return ParserService.GetProjectContent(p) ?? ParserService.DefaultProjectContent;
 			else
 				return ParserService.DefaultProjectContent;
 		}
 		
+		IProject FindProjectContainingFile()
+		{
+			if (ProjectService.OpenSolution != null) {
+				ProjectService.OpenSolution.FindProjectContainingFile(viewContent.PrimaryFileName);
+			}
+			return null;
+		}
+		
 		protected IClass GetDesignedClass()
 		{
 			Designer.Xaml.XamlDesignContext xamlContext = viewContent.DesignContext as Designer.Xaml.XamlDesignContext;