- 에러내용
You are mounting a new html component when a previous one has not first unmounted. It is an error to render more than one html component at a time and attributes and children of these components will likely fail in unpredictable ways. Please only render a single instance of<html>and if you need to mount a new one, ensure any previous ones have unmounted first.
- 결론
최상위의 layout에<html><body>가 있다면 하위 레이아웃 파일에는 해당 태그가 없어야 한다
:D