How to reproduce:
1. Open given Unity project
2. Enter "DisplayCancelableProgressBarBugScene"
3. Click on Main Camera
5. In inspector click on "Show bug error" button

Expected result: In editor should appear cancelable progress bar
Actual result: Error is thrown

Regression first introduced in: 2017.3.0a1
Reproducible with: 2018.2.0a5, 2018.1.0b12, 2017.4.0f1, 2017.3.2f1
Not reproducible with: 2017.2.2p1, 2017.1.3p2, 5.6.5p3

Note:
Error on 2018.2.0a5, 2018.1.0b12
EndLayoutGroup: BeginLayoutGroup must be called first.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

2017.4.0f1, 2017.3.2f1:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUILayoutUtility.BeginLayoutGroup (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options, System.Type layoutType) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:296)
UnityEditor.EditorGUILayout.BeginHorizontal (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7787)
UnityEditor.EditorGUILayout.BeginHorizontal (UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7767)
UnityEditor.InspectorWindow.AddComponentButton (UnityEditor.Editor[] editors) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1409)