fix SD-1876 - Removing reference after changing target framework causes InvalidOperationException

src/Main/Base/Project/Src/Project/MSBuildBasedProject.cs

@@ -979,6 +979,11 @@ namespace ICSharpCode.SharpDevelop.Project
 				
 				ClearFindFileCache();
 			}
+			
+			// refresh project browser to make sure references and other project items are still valid
+			// after TargetFramework or other properties changed. Fixes SD-1876
+			if (!isLoading)
+				ProjectBrowserPad.RefreshViewAsync();
 		}
 		
 		void IProjectItemListProvider.AddProjectItem(ProjectItem item)