Forms and controls must derive from a base class that supports designers. If you are deriving from something, such as an inherited form or control, within the project you are working in, you need to make sure the project is built by selecting Build Solution from the Build menu.
디자인 화면을 열자 위 메시지가 뜨고 에러 화면이 되버린 프로젝트 경우
빌드를 해주고 디자인 창을 닫고 다시 열면 이상없이 잘 열린다
