title: (Unity3D) Serializedobject target has been destroyed
body: <p>ERROR MESSAGE : </p>
  
  <blockquote>
    <p>"Serializedobject target has been destroyed. 
    UnityEngine.GUIUtility:ProcessEvent(Int32,IntPtr)"</p>
  </blockquote>
  
  <p>I get this error msg 8 times after pressing the play button. </p>
  
  <p>I upgrade from Unity 2017 to Unity 2018 beta (2018.1.0b7), and start getting this error 8 times every time I press the play button, the project runs normally but the sky box disappears from Skybox material in >lighting>scene bar</p>
  
  <p>Hope there is an easy way to fix it. </p>