Scene::setSceneSubtree crashes if called with null subTree parameter

Scene::setSceneSubtree crashes if called with null subTree parameter. This happens at least when trying to load non-scene urls with QSceneLoader. This case is trivially reproducable with assimp-cpp example by loading e.g. a png file.

I'm pretty sure this has also happened a couple of times when loading valid scenes in scene editor, but I can't repro that case on demand. If it happens, it is rare occurrence. There is a chance that those cases have actually been misclicks on wrong file, though.