Simple solution: Go to Unreal Projects'‘ProjectFolder’'\Saved\Config\WindowsEditor\EditorPerProjectUserSettings, Search [LandscapeEdit] See if you have a line like this ‘‘AlphaTextureName=/xxxxxxxx’’ and delete it then save.
It’s caused by moving or deleting the brush texture on landscape mode. So don’t forget to change the brush to something else before you delete or move the texture asset.