Fixed potential nullref when Destroy gets called repeatedly

Assets/Scripts/Core/Fsr3UpscalerContext.cs

@@ -125,9 +125,12 @@ namespace FidelityFX
             DestroyConstantBuffer(ref _spdConstantsBuffer);
             DestroyConstantBuffer(ref _upscalerConstantsBuffer);
 
+            if (_commandBuffer != null)
+            {
                 _commandBuffer.Dispose();
                 _commandBuffer = null;
             }
+        }
 
         public void Dispatch(Fsr3Upscaler.DispatchDescription dispatchParams)
         {