Enforce Content:CloneAssetFile() running on the main thread, to avoid a bug that occurs when a particle emitter is created from one of the templates due to the creation coming from the content thread.

Source/Engine/Content/Content.cpp

@@ -14,6 +14,7 @@
 #include "Engine/Engine/EngineService.h"
 #include "Engine/Platform/FileSystem.h"
 #include "Engine/Threading/Threading.h"
+#include "Engine/Threading/MainThreadTask.h"
 #include "Engine/Graphics/Graphics.h"
 #include "Engine/Engine/Time.h"
 #include "Engine/Engine/Globals.h"
@@ -688,8 +689,27 @@ bool Content::FastTmpAssetClone(const StringView& path, String& resultPath)
     return false;
 }
 
+class CloneAssetFileTask : public MainThreadTask
+{
+public:
+    StringView dstPath;
+    StringView srcPath;
+    Guid dstId;
+    bool* output;
+
+protected:
+    bool Run() override
+    {
+        *output = Content::CloneAssetFile(dstPath, srcPath, dstId);
+        return false;
+    }
+};
+
 bool Content::CloneAssetFile(const StringView& dstPath, const StringView& srcPath, const Guid& dstId)
 {
+    // Best to run this on the main thread to avoid clone conflicts.
+    if (IsInMainThread())
+    {
         PROFILE_CPU();
         ASSERT(FileSystem::AreFilePathsEqual(srcPath, dstPath) == false && dstId.IsValid());
 
@@ -784,6 +804,21 @@ bool Content::CloneAssetFile(const StringView& dstPath, const StringView& srcPat
                 storage->Reload();
             }
         }
+    }
+    else
+    {
+        CloneAssetFileTask* task = New<CloneAssetFileTask>();
+        task->dstId = dstId;
+        task->dstPath = dstPath;
+        task->srcPath = srcPath;
+
+        bool result = false;
+        task->output = &result;
+        task->Start();
+        task->Wait();
+
+        return result;
+    }
 
     return false;
 }