Return null pointer from mjs_firstChild and mjs_firstElement when the requested list is empty.

Fixes #1824.

PiperOrigin-RevId: 653758711
Change-Id: I6ede762cafd88dcbb0dfc9c2999b7aee24bd2e2a

src/user/user_model.cc

@@ -671,6 +671,13 @@ mjCBase* mjCModel::GetObject(mjtObj type, int id) {
 
 template <class T>
 static mjsElement* GetNext(std::vector<T*>& list, mjsElement* child) {
+  if (!child) {
+    if (list.empty()) {
+      return nullptr;
+    }
+    return list[0]->spec.element;
+  }
+
   // TODO: use id for direct indexing instead of a loop
   for (unsigned int i = 0; i < list.size()-1; i++) {
     if (list[i]->spec.element == child) {
@@ -696,37 +703,37 @@ mjsElement* mjCModel::NextObject(mjsElement* object, mjtObj type) {
 
   switch (type) {
     case mjOBJ_ACTUATOR:
-      return object ? GetNext(actuators_, object) : actuators_[0];
+      return GetNext(actuators_, object);
     case mjOBJ_SENSOR:
-      return object ? GetNext(sensors_, object) : sensors_[0];
+      return GetNext(sensors_, object);
     case mjOBJ_FLEX:
-      return object ? GetNext(flexes_, object) : flexes_[0];
+      return GetNext(flexes_, object);
     case mjOBJ_PAIR:
-      return object ? GetNext(pairs_, object) : pairs_[0];
+      return GetNext(pairs_, object);
     case mjOBJ_EXCLUDE:
-      return object ? GetNext(excludes_, object) : excludes_[0];
+      return GetNext(excludes_, object);
     case mjOBJ_EQUALITY:
-      return object ? GetNext(equalities_, object) : equalities_[0];
+      return GetNext(equalities_, object);
     case mjOBJ_TENDON:
-      return object ? GetNext(tendons_, object) : tendons_[0];
+      return GetNext(tendons_, object);
     case mjOBJ_NUMERIC:
-      return object ? GetNext(numerics_, object) : numerics_[0];
+      return GetNext(numerics_, object);
     case mjOBJ_TEXT:
-      return object ? GetNext(texts_, object) : texts_[0];
+      return GetNext(texts_, object);
     case mjOBJ_TUPLE:
-      return object ? GetNext(tuples_, object) : tuples_[0];
+      return GetNext(tuples_, object);
     case mjOBJ_KEY:
-      return object ? GetNext(keys_, object) : keys_[0];
+      return GetNext(keys_, object);
     case mjOBJ_MESH:
-      return object ? GetNext(meshes_, object) : meshes_[0];
+      return GetNext(meshes_, object);
     case mjOBJ_HFIELD:
-      return object ? GetNext(hfields_, object) : hfields_[0];
+      return GetNext(hfields_, object);
     case mjOBJ_SKIN:
-      return object ? GetNext(skins_, object) : skins_[0];
+      return GetNext(skins_, object);
     case mjOBJ_TEXTURE:
-      return object ? GetNext(textures_, object) : textures_[0];
+      return GetNext(textures_, object);
     case mjOBJ_MATERIAL:
-      return object ? GetNext(materials_, object) : materials_[0];
+      return GetNext(materials_, object);
     default:
       return nullptr;
   }