Fixed AnimationPlayer crashing Godot when its made the Scene Root

[ArshPanesar]

When an AnimationPlayer node is made the root of a scene, the node links on the animation track are broken and clicking on them will crash Godot. This happens because when it is made the scene root, the editor loses track of the AnimationPlayer's root_node, as the path to the root node gets broken.

By keeping a cached pointer to the AnimationPlayer's root_node, and using this copy when the editor can't find it, the track links can remain functional and avoid a crash.

Fixes Issue #91043 Works in 4.2.2-stable.