Only a ReferenceProjectItem should be Added to the ToolBox

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

@@ -47,6 +47,7 @@ namespace ICSharpCode.WpfDesign.AddIn
 
 		void ProjectService_ProjectItemAdded(object sender, ProjectItemEventArgs e)
 		{
+			if (e.ProjectItem is ReferenceProjectItem)
 				WpfToolbox.Instance.AddProjectDlls(this.Files[0]);
 		}