CRASH - When calling python function from invalid context/changing world

[Dariusz1989]

Hey

When I set scene via python api, the current scene may have an invalid world for some period of time...
``
@Persistent
def frame_change_handler(scene, depsgraph):
"""
Frame change handler, that redraws the atmosphere if and when
the scene has changed.
"""

if not scene.world or bpy.context.scene.world.psa_exposed.sun_object is None:
	return

Will crashm as bpy.context.scene has no world. 

if not scene.world or bpy.context.scene.world.psa_exposed.sun_object is None:
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

AttributeError: 'NoneType' object has no attribute 'psa_exposed'

We also need to check if current context scene word is valid...


**General information**
4.5.0

**To Reproduce**
Tricky...