Why doesn't glClearColor give effect in QtQuick2's scene initialization function?

Question

Qt

Hello. I'm learning Qt and OpenGL little by little. I am writing a small study project using QtQuick2 and OpenGL 4.3. Why glClearColor has no effect in the scene's initialize function? Although if you use it in the render function, then everything works. There is no such problem when using QWindow.
Project source code .