Why do I need to check nullptr when calling getNode()

Hi,
I just fixed a bug where I didn’t check for nullptr in onCollisionBegin when calling getNode() on the physicsBodies. Sometimes getNode() would return a nullptr and the game crashed.
My question is why does getNode() ever return nullptr? PhysicsBodies are always attached to a Node in my game so there shouldn’t be any bodies without an owner. Can someone who is more experienced with cocos2dx explain this to me please?