Right clicking on a Material and deleting it gives me these dialog box buttons.
Version 6 WIP (6.12.18317.12076, 2018-11-13)
Yes, it seems the RhinoMessageBox is broken on the Mac. Thanks for the report.
I created a YT for it
This will be fixed in the next RhinoWIP. SorryAboutThisInADiscourseReply.