Fix the error when clicking AnimationTree in the editor

[magian1127]

Fix #78052

[YuriSizov]

Thanks for opening a PR, but I don't think this is a correct solution. These are exactly the same checks, so if this fixes the issue that can only be possible by accident. If we need to be aware of running this code from the main thread only, it needs to be done explicitly.

I, unfortunately, can't tell without investigating what exactly depends on the main thread. So I can't point you to anything more specific.

[magian1127]

When singleton->tree is empty and AnimationTreeEditor.singleton is hidden, then AnimationTreeEditor is outside the main thread.
When singleton->tree is instantiated, AnimationTreeEditor is inside the main thread.
So changing the order of judgment will solve the problem.

[YuriSizov]

Right, but my point is that the reason for this order is not obvious from the code as it is. It needs to be made explicit why the checks are there and in this order. At least something like this would help:

	if (!singleton->tree || !singleton->is_visible()) {
		return Vector<String>(); // Not in the main thread, returning.
	}

	AnimationTree *tree = singleton->tree;
	if (!tree->has_node(tree->get_animation_player())) {
		return Vector<String>();
	}

[magian1127]

I followed your format and made some modifications, the deeper reasons, can only rely on others to solve it, my abilities are limited.

[YuriSizov]

Thanks!