1. Create a Terrain Object in the scene
2. Add a layer to the Terrain Object
3. Attempt to add the same layer to the Terrain Object

Actual Results: Occasionally there will be an ArgumentException
Expected Results: No Exception would occur

Exception:
ArgumentException: Getting control 0's position in a group with only 0 controls when doing repaint
Aborting
UnityEngine.GUILayoutGroup.GetNext () (at <931b26b61f444e3485195721b0227b20>:0)
UnityEngine.GUILayoutUtility.BeginLayoutGroup (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options, System.Type layoutType) (at <931b26b61f444e3485195721b0227b20>:0)
UnityEditor.EditorGUILayout.BeginHorizontal (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at <ee59ffdb2ee84322996931dbffc3a33c>:0)
UnityEditor.EditorGUILayout.BeginHorizontal (UnityEngine.GUILayoutOption[] options) (at <ee59ffdb2ee84322996931dbffc3a33c>:0)