What happened is that the last handle to an Actor shared_ptr was a
promoted weak_ptr. When the shared_ptr goes out of scope, the Actor dtor
is invoked. That involves removing the Actor collision object after
exclusively locking mCollisionWorldMutex. In this case, the lock was
already held in the outter scope of the promoted weak_ptr.
Reduce the scope of the mCollisionWorldMutex to never encompass the
lifetime of a promoted weak_ptr.