Fix call_group

`SceneTree#call_group` is crashing, but calling the methods it does manually works fine.

```v
// crashy
s.get_tree().call_group('mobs', 'queue_free')

// no crashy
mobs := s.get_tree().get_nodes_in_group('mobs')
for i in 0 .. mobs.size() {
	mob := mobs.get(i)
	mut node := gd.Node.new()
	node.from_variant(mob)
	node.queue_free()
}
```