let fake_entity = _entity_from_index.call(0);
let fake_entity_valid = _make_invalid_entity.call();
assert_throws(||{
world.insert_children.call(fake_entity_valid, 0, [fake_entity_valid]);
}, "Missing or invalid entity");
let entity = world.spawn_.call();
assert_throws(||{
world.insert_children.call(entity, 0, [fake_entity]);
}, "Are you trying to use an entity in a callback in which it's unavailable?");