QOpenGLWidget content not visible, only black renders

I have a (subclass of a) QOpenGLWidget, with a (subclass of a) QMainWindow as its parent. When I run my application, I should be seeing content rendered onto this widget. However, I only see black, as if the content failed to render. I don't know if glClear is succeeding or not.

I also get this error a lot:

QOpenGLContext::swapBuffers() called with non-exposed window, behavior is undefined

The source file for which this is relevant is here https://github.com/JesseTG/BALLS/blob/master/BALLS/BALLS/ui/BallsCanvas.cpp

For now, I can work around this by passing -style=fusion as an argument to my program, but I'd rather not make other people do this.