### Godot version

v4.1.rc2.official [46424488e]

### System information

Godot v4.1.rc2 - Windows 10.0.22621 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3060 (NVIDIA; 31.0.15.3623) - AMD Ryzen 7 5700G with Radeon Graphics (16 Threads)

### Issue description

When setting the AnimationTree to active, Godot crashes.

### Steps to reproduce

https://github.com/godotengine/godot/assets/46705245/dbe5eb81-1357-43e8-ac80-a4eb97eea9fc

### Minimal reproduction project

[Crash.zip](https://github.com/godotengine/godot/files/11945826/Crash.zip)

return len_fade_from;
}
float AnimationNodeStateMachinePlayback::get_fading_time() const {
	return fading_time;
}
float AnimationNodeStateMachinePlayback::get_fading_pos() const {
	return fading_pos;
}
void AnimationNodeStateMachinePlayback::_clear_path_children(AnimationTree *p_tree, AnimationNodeStateMachine *p_state_machine, bool p_test_only) {
	List<AnimationNode::ChildNode> child_nodes;
	p_state_machine->get_child_nodes(&child_nodes);
	for (int i = 0; i < child_nodes.size(); i++) {
		Ref<AnimationNodeStateMachine> anodesm = child_nodes[i].node;
		if (anodesm.is_valid() && anodesm->get_state_machine_type() == AnimationNodeStateMachine::STATE_MACHINE_TYPE_GROUPED) {
			Ref<AnimationNodeStateMachinePlayback> playback = p_tree->get(base_path + child_nodes[i].name + "/playback");
			ERR_FAIL_COND(!playback.is_valid());
			playback->_set_base_path(base_path + child_nodes[i].name + "/");
			if (p_test_only) {
				playback = playback->duplicate();
			}
			playback->path.clear();
			playback->_clear_path_children(p_tree, anodesm.ptr(), p_test_only);
			if (current != child_nodes[i].name) {
				playback->_start(anodesm.ptr()); 
			}
		}
	}
}
void AnimationNodeStateMachinePlayback::_start_children(AnimationTree *p_tree, AnimationNodeStateMachine *p_state_machine, const String &p_path, bool p_test_only) {