Exceptions and broken Inspector window after adjusting additional settings in Avatar Editor

Select an Avatar asset and go to Configure Avatar.
Go to the Muscles & Settings tab.
Change any of the Additional Settings values and click Apply.

A lot of Exceptions get logged in the Console and the Inspector window stops rendering anything. The only way to get out of this mode is to reload the Editor layout.

Not reproducible with 2018.1.0a7 (093ff45d2bc9), so likely introduced somewhere in one of the 2018.1 beta builds.

Exceptions look like these:

Unexpected top level layout group! Missing GUILayout.EndScrollView/EndVertical/EndHorizontal?
UnityEngine.DebugLogHandler:Internal_Log(LogType, String, Object)
UnityEngine.DebugLogHandler:LogFormat(LogType, Object, String, Object[])
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:LogWarning(Object)
UnityEditor.InspectorWindow:DrawEditor(Editor[], Int32, Boolean, Boolean&, Rect&) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1333)
UnityEditor.InspectorWindow:DrawEditors(Editor[]) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1073)
UnityEditor.InspectorWindow:OnGUI() (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:413)
System.Reflection.MonoMethod:InternalInvoke(Object, Object[], Exception&)
System.Reflection.MonoMethod:Invoke(Object, BindingFlags, Binder, Object[], CultureInfo) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
System.Reflection.MethodBase:Invoke(Object, Object[]) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView:Invoke(String, Object) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:291)
UnityEditor.HostView:Invoke(String) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:284)
UnityEditor.HostView:InvokeOnGUI(Rect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:251)
UnityEditor.DockArea:OldOnGUI() (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:386)
UnityEngine.Experimental.UIElements.IMGUIContainer:DoOnGUI(Event) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:203)
UnityEngine.Experimental.UIElements.IMGUIContainer:HandleIMGUIEvent(Event) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:355)
UnityEngine.Experimental.UIElements.EventDispatcher:PropagateToIMGUIContainer(VisualElement, EventBase, VisualElement) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:399)
UnityEngine.Experimental.UIElements.EventDispatcher:PropagateToIMGUIContainer(VisualElement, EventBase, VisualElement) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:411)
UnityEngine.Experimental.UIElements.EventDispatcher:DispatchEvent(EventBase, IPanel) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:380)
UnityEngine.Experimental.UIElements.UIElementsUtility:DoDispatch(BaseVisualElementPanel) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:224)
UnityEngine.Experimental.UIElements.UIElementsUtility:ProcessEvent(Int32, IntPtr) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:77)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr) (at /Users/builduser/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:184)

(Filename: /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs Line: 1333)

NullReferenceException: Object reference not set to an instance of an object
at UnityEditor.AvatarMuscleEditor.OnInspectorGUI () [0x0003b] in /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarMuscleEditor.cs:606
at UnityEditor.AvatarEditor.EditingGUI () [0x000c5] in /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:440
at UnityEditor.AvatarEditor.OnInspectorGUI () [0x00026] in /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarEditor.cs:378
at UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[] editors, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) [0x0040b] in /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1298
UnityEngine.DebugLogHandler:Internal_LogException(Exception, Object)
UnityEngine.DebugLogHandler:LogException(Exception, Object)
UnityEngine.Logger:LogException(Exception, Object)
UnityEngine.Debug:LogException(Exception)
UnityEditor.InspectorWindow:DrawEditor(Editor[], Int32, Boolean, Boolean&, Rect&) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1305)
UnityEditor.InspectorWindow:DrawEditors(Editor[]) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1073)
UnityEditor.InspectorWindow:OnGUI() (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:413)
System.Reflection.MonoMethod:InternalInvoke(Object, Object[], Exception&)
System.Reflection.MonoMethod:Invoke(Object, BindingFlags, Binder, Object[], CultureInfo) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
System.Reflection.MethodBase:Invoke(Object, Object[]) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView:Invoke(String, Object) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:291)
UnityEditor.HostView:Invoke(String) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:284)
UnityEditor.HostView:InvokeOnGUI(Rect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:251)
UnityEditor.DockArea:OldOnGUI() (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:386)
UnityEngine.Experimental.UIElements.IMGUIContainer:DoOnGUI(Event) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:203)
UnityEngine.Experimental.UIElements.IMGUIContainer:HandleIMGUIEvent(Event) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:358)
UnityEngine.Experimental.UIElements.EventDispatcher:PropagateToIMGUIContainer(VisualElement, EventBase, VisualElement) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:399)
UnityEngine.Experimental.UIElements.EventDispatcher:PropagateToIMGUIContainer(VisualElement, EventBase, VisualElement) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:411)
UnityEngine.Experimental.UIElements.EventDispatcher:DispatchEvent(EventBase, IPanel) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:380)
UnityEngine.Experimental.UIElements.UIElementsUtility:DoDispatch(BaseVisualElementPanel) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:224)
UnityEngine.Experimental.UIElements.UIElementsUtility:ProcessEvent(Int32, IntPtr) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:77)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr) (at /Users/builduser/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:184)

(Filename: /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Avatar/AvatarMuscleEditor.cs Line: 606)