Make sure Academy is initialized in ProjectSettingsOverrides.Awake()

Project/Assets/ML-Agents/Examples/SharedAssets/Scripts/ProjectSettingsOverrides.cs

             Physics.defaultSolverIterations = solverIterations;
             Physics.defaultSolverVelocityIterations = solverVelocityIterations;
 
+            // Make sure the Academy singleton is initialized first, since it will create the SideChannels.
+            var academy = Academy.Instance;
             SideChannelUtils.GetSideChannel<FloatPropertiesChannel>().RegisterCallback("gravity", f => { Physics.gravity = new Vector3(0, -f, 0); });
         }
 
             Physics.defaultSolverVelocityIterations = m_OriginalSolverVelocityIterations;
         }
     }
-}
+}