Regression - can't repro in 5.6 (5.6.1f1 and 5.6.1p2)
reproduces on 2017.1.0 (checked on 2017.1.0a4 and 2017.1.0b8) and on 2017.2.0a1

How we can reproduce it using the example you attached
1 Create new project
2 In GameView "header" set Display 2 (or any other except of Display 1)
3 Left mouse button Click a bit higher than bottom border of Display 2 dropdown (see image in comments)

Actual results:
2 exceptions appears in the Console:
"ArgumentException: You can only call GUI functions from inside OnGUI.
UnityEngine.GUIUtility.CheckOnGUI () "
"NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUIUtility.EndGUI (Int32 layoutType)"

Expected results:
There should not be any exceptions or errors