Terrain Brush Disappeared

I can no longer see any brushes in the default terrain view. Does anyone have a clue as to what is going on and how I can fix this bug? Currently using 2018.3.0b1

UnassignedReferenceException: The variable m_Mask of Brush has not been assigned.
You probably need to assign the m_Mask variable of the Brush script in the inspector.
UnityEditor.Brush.GenerateBrushTexture (UnityEngine.Texture2D mask, UnityEngine.AnimationCurve falloff, System.Single radiusScale, System.Int32 width, System.Int32 height, System.Boolean isThumbnail) (at C:/buildslave/unity/build/Modules/TerrainEditor/Brush/Brush.cs:116)
UnityEditor.Brush.UpdateThumbnail () (at C:/buildslave/unity/build/Modules/TerrainEditor/Brush/Brush.cs:63)
UnityEditor.Brush.get_thumbnail () (at C:/buildslave/unity/build/Modules/TerrainEditor/Brush/Brush.cs:69)
UnityEditor.BrushList.GUIContentFromBrush (UnityEditor.Brush[] brushes) (at C:/buildslave/unity/build/Modules/TerrainEditor/Brush/BrushList.cs:238)
UnityEditor.BrushList.BrushSelectionGrid (System.Int32 selected, UnityEditor.Brush[] brushes, System.Int32 approxSize, UnityEngine.GUIStyle style, UnityEngine.GUIContent emptyString, System.Boolean& doubleClick) (at C:/buildslave/unity/build/Modules/TerrainEditor/Brush/BrushList.cs:206)
UnityEditor.BrushList.ShowGUI () (at C:/buildslave/unity/build/Modules/TerrainEditor/Brush/BrushList.cs:119)
UnityEditor.TerrainInspector.ShowBrushes (System.Int32 spacing) (at C:/buildslave/unity/build/Modules/TerrainEditor/TerrainInspector.cs:1405)
UnityEditor.Experimental.TerrainAPI.OnInspectorGUIContext.ShowBrushesGUI (System.Int32 spacing) (at C:/buildslave/unity/build/Modules/TerrainEditor/PaintTools/TerrainPaintTool.cs:120)
UnityEditor.Experimental.TerrainAPI.PaintTextureTool.OnInspectorGUI (UnityEngine.Terrain terrain, UnityEditor.Experimental.TerrainAPI.IOnInspectorGUI editContext) (at C:/buildslave/unity/build/Modules/TerrainEditor/PaintTools/PaintTextureTool.cs:139)
UnityEditor.TerrainInspector.ShowPaint () (at C:/buildslave/unity/build/Modules/TerrainEditor/TerrainInspector.cs:1398)
UnityEditor.TerrainInspector.OnInspectorGUI () (at C:/buildslave/unity/build/Modules/TerrainEditor/TerrainInspector.cs:1752)
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:1599)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Edit: I deleted the custom brushes that I had added, and the original brushes have returned.

I had the same problem. I tried upgrading to 2018.3.0b3 but the problem persisted. It was localized to that project. I created a new scene with a new terrain and it still had the same problem. I believe I may have caused it by moving the terrain object, or one of two custom brushes into a subfolder in my project window.

I created a new project, and the new project appears to be okay. Don’t know how to fix the existing project.

Hello, sorry for the translation “made in Google” but I’m French …
This is actually a bug related to creating a brush. To solve the problem, search the project folder for “* .brush” files and either delete them or rename them (if you do not know the name of the brush and there are several). You can do this when Unity works, once the brush “culprit” deleted or rename, the field editor returns to normal.