Thanks for your answer. It's working now.

Hi everyone. I'm trying to display a very simple menu. But everytime I play the scene, it crashed with this message : NullReferenceException: Object reference not set to an instance of an object UnityEditor.DockArea.OnGUI () (at C:/BuildAgent/work/d3d49558e4d408f4/Editor/Mono/GUI/DockArea.cs:585) Sy...