What does this console error means ? NullReferenceException ?

Please, I'm getting this error message in my console : "NullReferenceException: Object reference not set to an instance of an object

UnityEditor.GameView.Maximize () "

-I don't know what it means, do you know, please ?

-How can I fix it?

-It comes from the tutorial "FPS_Tutorial_Completed" available there: FPS_tutorial

it means that, you are accessing a null reference variable in your script. you should either set it in the inspector or at runtime. if it's FPS tutorial so i think there is something that you should set it up in inspector but you did not.