"NullReferenceException: Object reference not set to an instance of an object UnityEditor.TerrainInspector.ShowRenderingLayerMask (System.Boolean useMiniStyle)" are thrown continuously when playing with terrain settings.
Steps to repro:
1. Create a new project.
2. Hierarchy > Right-click > 3D Object > Terrain
3. Inspector > Terrain > Terrain settings.

Actual Result:
NullReference exception continuously thrown when playing with terrain settings.
Expected Result:
Nullrefence exceptions are not received

Reproducible in:
2019.3.0a10, 2019.3.0a9

Working Fine in:
2019.3.0a8, 2019.2.0b10

Environment:
Windows and Mac.

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.TerrainInspector.ShowRenderingLayerMask (System.Boolean useMiniStyle) (at /Users/builduser/buildslave/unity/build/Modules/TerrainEditor/TerrainInspector.cs:1456)
UnityEditor.TerrainInspector.ShowSettings () (at /Users/builduser/buildslave/unity/build/Modules/TerrainEditor/TerrainInspector.cs:1447)
UnityEditor.TerrainInspector.OnInspectorGUI () (at /Users/builduser/buildslave/unity/build/Modules/TerrainEditor/TerrainInspector.cs:2011)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass53_0.<CreateIMGUIInspectorFromEditor>b__0 () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorElement.cs:494)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)