As far as I can tell, with Launchbox 12.15, this can still happen, at least when using drag and drop folders. What seemed to get around it for me was instead of doing drag and drop, do Tools > Import > MS-DOS games, then add the folder(s) there. I would get a message in the log like:
2022-10-01 12:56:03 PM FIRST CHANCE EXCEPTION: Value cannot be null. (Parameter 'path2')
at System.IO.Path.Combine(String path1, String path2)
2022-10-01 12:56:03 PM ERROR: Encountered error while building MS-DOS import task via Drag/Drop menu action
2022-10-01 12:56:03 PM EXCEPTION IGNORED: Value cannot be null. (Parameter 'path2')
at System.IO.Path.Combine(String path1, String path2)
at Unbroken.LaunchBox.Windows.Desktop.ViewModels.DragDropGameSelectViewModel.ConnectJmsMethod(Object , Object )
at Unbroken.LaunchBox.Windows.Desktop.ViewModels.DragDropGameSelectViewModel.GetDosTask()
