Added check before attempting to remove actor's collision object from world

apps/openmw/mwphysics/actor.cpp

@@ -71,8 +71,11 @@ void Actor::enableCollisionBody(bool collision)
 }
 
 void Actor::updateCollisionMask()
+{
+    if (mCollisionObject.get()->getWorldArrayIndex() >= 0)
     {
         mCollisionWorld->removeCollisionObject(mCollisionObject.get());
+    }
     int collisionMask = CollisionType_World | CollisionType_HeightMap;
     if (mExternalCollisionMode)
         collisionMask |= CollisionType_Actor | CollisionType_Projectile | CollisionType_Door;