Avoid crashing in QOpenGLWidget::metrics

When creating child widgets in the derived class' constructor, the screen
cannot be retrieved via window() since it still returns null. Handle this
case properly.

Change-Id: I65bf7f9c338ce8a3ddab8ad7886cfe2f1f253c82
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>