This error occurs when you return from Play mode and had a brush selected while entering Play mode. Unity 2019.2.2f1
MissingReferenceException: The object of type 'CSGBrush' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
RealtimeCSG.EditModeSelectionGUI.OnInspectorGUI (UnityEditor.Editor editor, UnityEngine.Object[] targets) (at Assets/Plugins/RealtimeCSG/Editor/Scripts/View/GUI/EditModeGUI/EditModeSelection.GUI.cs:267)
EditModeToolWindowEditor.OnInspectorGUI () (at Assets/Plugins/RealtimeCSG/Editor/Scripts/View/GUI/EditModeGUI/EditModeToolWindow.Editor.cs:15)
UnityEditor.UIElements.InspectorElement+<CreateIMGUIInspectorFromEditor>c__AnonStorey1.<>m__0 () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorElement.cs:501)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
This error occurs when you return from Play mode and had a brush selected while entering Play mode. Unity 2019.2.2f1