### 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)

if (base_path.is_empty()) {
		return Ref<AnimationNodeStateMachine>();
	}
	Vector<String> split = base_path.split("/");
	ERR_FAIL_COND_V_MSG(split.size() < 3, Ref<AnimationNodeStateMachine>(), "Path is too short.");
	split = split.slice(1, split.size() - 2);
	Ref<AnimationNode> root = p_tree->get_tree_root();
	ERR_FAIL_COND_V_MSG(root.is_null(), Ref<AnimationNodeStateMachine>(), "There is no root AnimationNode in AnimationTree: " + String(p_tree->get_name()));
	String anodesm_path = String("/").join(split);
	Ref<AnimationNodeStateMachine> anodesm = !anodesm_path.size() ? root : root->find_node_by_path(anodesm_path);
	ERR_FAIL_COND_V_MSG(anodesm.is_null(), Ref<AnimationNodeStateMachine>(), "Can't get state machine with path: " + anodesm_path);
	return anodesm;
}
Ref<AnimationNodeStateMachineTransition> AnimationNodeStateMachinePlayback::_get_group_start_transition() const {