Fix "UnassignedReferenceException" when playing in Editor

Assets/Scripts/LightEstimation.cs

             m_CameraManager.frameReceived += FrameChanged;
 
         // Disable the arrow to start; enable it later if we get directional light info
-        arrow?.gameObject.SetActive(false);
+        if (arrow)
+        {
+            arrow?.gameObject.SetActive(false);
+        }
         Application.onBeforeRender += OnBeforeRender;
     }