Fix AnimationPlayer crash when it's made the scene root

When an AnimationPlayer is made root of a scene,
the track links may become broken and clicking on
them will crash.

Current master branch also breaks node links when
AnimationPlayer is made scene root, and can also
crash the engine if another node was made scene
root prior to the AnimationPlayer.

This happens because when made root, the editor
loses track of AnimPlayer's root node. By keeping
a copy of the AnimPlayer's root_node, the track
links remain functional.

Fixes godotengine#91043.

editor/plugins/animation_player_editor_plugin.cpp

@@ -402,7 +402,17 @@ void AnimationPlayerEditor::_animation_selected(int p_which) {
 
 			track_editor->set_animation(anim, animation_is_readonly);
 			Node *root = player->get_node_or_null(player->get_root_node());
-			if (root) {
+
+			// Player shouldn't access parent if it's the scene root.
+			if (!root || (player == get_tree()->get_edited_scene_root() && player->get_root_node() == SceneStringName(path_pp))) {
+				NodePath cached_root_path = player->get_path_to(get_cached_root_node());
+				if (player->get_node_or_null(cached_root_path) != nullptr) {
+					player->set_root_node(cached_root_path);
+				} else {
+					player->set_root_node(SceneStringName(path_pp)); // No other choice, preventing crash.
+				}
+			} else {
+				cached_root_node_id = root->get_instance_id(); // Caching as `track_editor` can lose track of player's root node.
 				track_editor->set_root(root);
 			}
 		}
@@ -1886,6 +1896,10 @@ AnimationMixer *AnimationPlayerEditor::fetch_mixer_for_library() const {
 	return original_node;
 }
 
+Node *AnimationPlayerEditor::get_cached_root_node() const {
+	return Object::cast_to<Node>(ObjectDB::get_instance(cached_root_node_id));
+}
+
 bool AnimationPlayerEditor::_validate_tracks(const Ref<Animation> p_anim) {
 	bool is_valid = true;
 	if (!p_anim.is_valid()) {

editor/plugins/animation_player_editor_plugin.h

@@ -52,6 +52,7 @@ class AnimationPlayerEditor : public VBoxContainer {
 	AnimationPlayerEditorPlugin *plugin = nullptr;
 	AnimationMixer *original_node = nullptr; // For pinned mark in SceneTree.
 	AnimationPlayer *player = nullptr; // For AnimationPlayerEditor, could be dummy.
+	ObjectID cached_root_node_id;
 	bool is_dummy = false;
 
 	enum {
@@ -253,6 +254,7 @@ class AnimationPlayerEditor : public VBoxContainer {
 	AnimationMixer *get_editing_node() const;
 	AnimationPlayer *get_player() const;
 	AnimationMixer *fetch_mixer_for_library() const;
+	Node *get_cached_root_node() const;
 
 	static AnimationPlayerEditor *get_singleton() { return singleton; }