Null exception reference occurs on applying humanoid configuration when cursor is hovering over import pop-up

Steps to Repro:
1. Open attached project or import humanoid model > Project window
2. select model > inspector > import settings > rig
3. Animation type > humanoid > apply > configure
4. Change few settings > done > apply changes
5. Click on already selected GameObject in hierarchy view > observe console for errors

Occurring On:
2018.1.0b5

Working with:
2018.1.0b4, 2017.3.0f3

Environment:
Occurring only on Mac

Stack Trace:
Error 1: EndLayoutGroup: BeginLayoutGroup must be called first.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Error 2:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUILayoutUtility.BeginLayoutGroup (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options, System.Type layoutType) (at /Users/builduser/buildslave/unity/build/Modules/IMGUI/GUILayoutUtility.cs:317)
UnityEditor.EditorGUILayout.BeginHorizontal (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7797)
UnityEditor.InspectorWindow.DrawPreviewAndLabels () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:689)
UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:449)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:291)
UnityEditor.HostView.Invoke (System.String methodName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:284)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:251)