Repro steps:
- open this project and the Sample Scene
- use the tile palette to paint some tiles, remove some tiles and go into play mode
- ramdomly, the hierarcy window will disappear and errors will pop up in the console:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.SceneHierarchy.get_hasSortMethods () (at C:/buildslave/unity/build/Editor/Mono/SceneHierarchy.cs:155)
UnityEditor.SceneHierarchyWindow.DoToolbarLayout () (at C:/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:189)
UnityEditor.SceneHierarchyWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneHierarchyWindow.cs:132)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <ac823e2bb42b41bda67924a45a0173c3>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <ac823e2bb42b41bda67924a45a0173c3>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <ac823e2bb42b41bda67924a45a0173c3>:0)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:335)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:329)
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition, UnityEngine.Rect viewRect) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:303)
UnityEditor.DockArea.DrawView (UnityEngine.Rect viewRect, UnityEngine.Rect dockAreaRect, System.Boolean customBorder, System.Boolean floatingWindow, System.Boolean isBottomTab) (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:360)
UnityEditor.DockArea.OldOnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:319)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:244)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

not reproduced in Unity Editor 2018.3.0a9
reproduced in Unity Editor 2018.3.0a10 and a11