Reference to TerrainLayer exists after deleting the Terrain Layer from project/disk

Repro:
1. Create/open project in 2018.3.0b6
2. Create/open scene
3. Create/select Terrain
4. Select the Paint Texture Tool
5. Add a new Terrain Layer
6. Paint on Terrain using that Terrain Layer
7. Select Terrain Layer in the Project window
8. Delete it
9. Select Terrain again
10. Select Raise/Lower Height tool and paint with that on the Terrain
11. "Fixed" upon saving, though the null ref terrain layer still shows up in the Inspector GUI for Terrain

Errors in Console:
1. "NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Experimental.TerrainAPI.PaintTextureTool.DrawFoldoutEditor (UnityEditor.Editor editor, System.Int32 controlId, System.Boolean& visible) (at C:/buildslave/unity/build/Modules/TerrainEditor/PaintTools/PaintTextureTool.cs:73)
UnityEditor.Experimental.TerrainAPI.PaintTextureTool.OnInspectorGUI (UnityEngine.Terrain terrain, UnityEditor.Experimental.TerrainAPI.IOnInspectorGUI editContext) (at C:/buildslave/unity/build/Modules/TerrainEditor/PaintTools/PaintTextureTool.cs:150)
UnityEditor.TerrainInspector.ShowPaint () (at C:/buildslave/unity/build/Modules/TerrainEditor/TerrainInspector.cs:1458)
UnityEditor.TerrainInspector.OnInspectorGUI () (at C:/buildslave/unity/build/Modules/TerrainEditor/TerrainInspector.cs:1813)
UnityEditor.InspectorWindow.DoOnInspectorGUI (System.Boolean rebuildOptimizedGUIBlock, UnityEditor.Editor editor, System.Boolean wasVisible, UnityEngine.Rect& contentRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1598)
UnityEditor.PopupCallbackInfo:SetEnumValueDelegate(Object, String[], Int32)"