Repros:

- use attach bug.unitypackages
- Load scene named Bug
- Click Editor Play and notice errors:

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUILayoutUtility.LayoutFromEditorWindow () (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:168)
UnityEngine.GUIUtility.EndGUI (Int32 layoutType) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUIUtility.cs:439)

ArgumentException: Getting control 1's position in a group with only 1 controls when doing Repaint
Aborting
UnityEngine.GUILayoutGroup.GetNext () (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/LayoutGroup.cs:114)
UnityEngine.GUILayoutUtility.BeginLayoutGroup (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options, System.Type layoutType) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:297)
UnityEngine.GUILayout.BeginVertical (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:308)
UnityEngine.GUILayout.BeginVertical (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:299)
UnityEditor.HostView.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:80)

Assertion failed on expression: 'device.IsInsideFrame()'

- Issue happen with any sort of events (created in the importer or Animation Windows)

not repro in...
Version 5.5.1p3 (828893732fe0)
Branch: 5.5/patch-release