get_child error when duplicating .GLB model nodes with animations

[retexcraft]

Tested versions

reproducible in 4.3 stable
not reproducible in 4.2 stable
not tested in other builds

System information

Godot v4.3.stable - Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1070 (NVIDIA; 31.0.15.5161) - Intel(R) Core(TM) i7-7700 CPU @ 3.60GHz (8 Threads)

Issue description

when attempting to duplicate() a GLB node with animations under its animation player it is unable to understand the children and produces this error

E 0:00:00:0862   node_3d.gd:5 @ _ready(): Index p_index = 1 is out of bounds ((int)data.children_cache.size() = 1).
  <C++ Source>   scene/main/node.cpp:1688 @ get_child()
  <Stack Trace>  node_3d.gd:5 @ _ready()

that also causes the next error in the debugger

E 0:00:00:0863   node_3d.gd:5 @ _ready(): Child node disappeared while duplicating.
  <C++ Error>    Parameter "copy_child" is null.
  <C++ Source>   scene/main/node.cpp:2926 @ _duplicate_properties()
  <Stack Trace>  node_3d.gd:5 @ _ready()

Steps to reproduce

import a .GLB model with a animation into a scene and call the duplicate function on it

Minimal reproduction project (MRP)

glb-duplicate-test.zip

[matheusmdx]

Bisecting points to #91677 as the culprit, @KoBeWi

This looks not related to animation, but a problem with internal nodes, while duplicating this part discards the Skeleton3D internal node for the new node:

godot/scene/main/node.cpp

Lines 2715 to 2722 in 61598c5

	for (List<const Node *>::Element *N = node_tree.front(); N; N = N->next()) {
	for (int i = 0; i < N->get()->get_child_count(); ++i) {
	Node *descendant = N->get()->get_child(i);
	if (!descendant->get_owner()) {
	continue; // Internal nodes or nodes added by scripts.
	}

Which results in a difference in child count here, p_original has two childs (the internal bone node and the cube) while p_copy has one (the cube):

godot/scene/main/node.cpp

Lines 2954 to 2958 in 61598c5

	for (int i = 0; i < p_original->get_child_count(); i++) {
	Node *copy_child = p_copy->get_child(i);
	ERR_FAIL_NULL_MSG(copy_child, "Child node disappeared while duplicating.");
	_duplicate_properties(p_root, p_original->get_child(i), copy_child, p_flags);
	}

Commenting line 2720 preserves the internal node and stop the error but probably is not the correct fix, but at least give some idea what's going on

[KoBeWi]

Bisecting points to #91677 as the culprit

#91677 only added a null check. The MRP crashes otherwise.

[matheusmdx]

Bisecting points to #91677 as the culprit

#91677 only added a null check. The MRP crashes otherwise.