Steps to reproduce:
1) Open the attached project.
2) Select 'hexagonAll_sheet.png' from the Assets folder.
3) Go to Sprite Editor and press Slice > Slice.

Errors appear in the console:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUILayout.Space (Single pixels) (at /home/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:246)

InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)

Reproduced on:
5.4.0b23

Did not reproduce on:
5.4.0b21, 5.4.0b18