Fix spawning nested prefab with different root actor

#1138

Source/Engine/Level/Prefabs/Prefab.cpp

@@ -2,6 +2,7 @@
 
 #include "Prefab.h"
 #include "Engine/Serialization/JsonTools.h"
+#include "Engine/Content/Content.h"
 #include "Engine/Content/Factories/JsonAssetFactory.h"
 #include "Engine/Core/Log.h"
 #include "Engine/Level/Prefabs/PrefabManager.h"
@@ -21,6 +22,36 @@ Prefab::Prefab(const SpawnParams& params, const AssetInfo* info)
 {
 }
 
+Guid Prefab::GetRootObjectId() const
+{
+    ASSERT(IsLoaded());
+    ScopeLock lock(Locker);
+
+    // Root is always the first but handle case when prefab root was reordered in the base prefab while the nested prefab has still the old state
+    // TODO: resave and force sync prefabs during game cooking so this step could be skipped in game
+    int32 objectIndex = 0;
+    if (NestedPrefabs.HasItems())
+    {
+        const auto& data = *Data;
+        const Guid basePrefabId = JsonTools::GetGuid(data[objectIndex], "PrefabID");
+        if (const auto basePrefab = Content::Load<Prefab>(basePrefabId))
+        {
+            const Guid basePrefabRootId = basePrefab->GetRootObjectId();
+            for (int32 i = 0; i < ObjectsCount; i++)
+            {
+                const Guid prefabObjectId = JsonTools::GetGuid(data[i], "PrefabObjectID");
+                if (prefabObjectId == basePrefabRootId)
+                {
+                    objectIndex = i;
+                    break;   
+                }
+            }
+        }
+    }
+
+    return ObjectsIds[objectIndex];
+}
+
 Actor* Prefab::GetDefaultInstance()
 {
     ScopeLock lock(Locker);