Fixed bullet server null pointer check

Fixed bullet server null pointer check

modules/bullet/bullet_physics_server.cpp

@@ -798,7 +798,7 @@ bool BulletPhysicsServer::body_is_omitting_force_integration(RID p_body) const {
 void BulletPhysicsServer::body_set_force_integration_callback(RID p_body, Object *p_receiver, const StringName &p_method, const Variant &p_udata) {
 	RigidBodyBullet *body = rigid_body_owner.get(p_body);
 	ERR_FAIL_COND(!body);
-	body->set_force_integration_callback(p_receiver->get_instance_id(), p_method, p_udata);
+	body->set_force_integration_callback(p_receiver ? p_receiver->get_instance_id() : ObjectID(0), p_method, p_udata);
 }
 
 void BulletPhysicsServer::body_set_ray_pickable(RID p_body, bool p_enable) {