Check for valid stack in set_bone_index

Resolves #54235

scene/resources/skeleton_modification_2d_lookat.cpp

@@ -241,7 +241,7 @@ int SkeletonModification2DLookAt::get_bone_index() const {
 void SkeletonModification2DLookAt::set_bone_index(int p_bone_idx) {
 	ERR_FAIL_COND_MSG(p_bone_idx < 0, "Bone index is out of range: The index is too low!");
 
-	if (is_setup) {
+	if (is_setup && stack) {
 		if (stack->skeleton) {
 			ERR_FAIL_INDEX_MSG(p_bone_idx, stack->skeleton->get_bone_count(), "Passed-in Bone index is out of range!");
 			bone_idx = p_bone_idx;