Fix crash in particles system when assets gets loading/unloaded while async jobs are active

Source/Engine/Particles/Particles.cpp

@@ -111,6 +111,7 @@ namespace ParticleManagerImpl
 using namespace ParticleManagerImpl;
 
 TaskGraphSystem* Particles::System = nullptr;
+ConcurrentSystemLocker Particles::SystemLocker;
 bool Particles::EnableParticleBufferPooling = true;
 float Particles::ParticleBufferRecycleTimeout = 10.0f;
 
@@ -139,6 +140,8 @@ class ParticlesSystem : public TaskGraphSystem
 {
 public:
     float DeltaTime, UnscaledDeltaTime, Time, UnscaledTime;
+    bool Active;
+
     void Job(int32 index);
     void Execute(TaskGraph* graph) override;
     void PostExecute(TaskGraph* graph) override;
@@ -1390,6 +1393,10 @@ void ParticlesSystem::Execute(TaskGraph* graph)
 {
     if (UpdateList.Count() == 0)
         return;
+    Active = true;
+
+    // Ensure no particle assets can be reloaded/modified during async update
+    Particles::SystemLocker.Begin(false);
 
     // Setup data for async update
     const auto& tickData = Time::Update;
@@ -1406,8 +1413,13 @@ void ParticlesSystem::Execute(TaskGraph* graph)
 
 void ParticlesSystem::PostExecute(TaskGraph* graph)
 {
+    if (!Active)
+        return;
     PROFILE_CPU_NAMED("Particles.PostExecute");
 
+    // Cleanup
+    Particles::SystemLocker.End(false);
+    Active = false;
     UpdateList.Clear();
 
 #if COMPILE_WITH_GPU_PARTICLES