Thunderbird 60.0b7 mbox to maildir conversion question

Firstly, thanks for all the hard work on this project, I’ve been a Thunderbird fan for some years now.
I tried the conversion of folders from mbox to maildir using this beta yesterday, and although it seemed to finish converting the folders for the account in question, there was no automatic Thunderbird restart at the end of the conversion. Rather, it displayed a dialog box with “conversion complete”, the green bar was all the way to the right, and a cancel button was displayed in the dialog box. But no “Ok” button, or any way to exit.
I checked the profile folders, and there did seem to be maildir folders created, so obviously something worked.
So, what was I doing wrong to have got this far and then… no re-start?
(By the way, I did try re-starting Thunderbird myself after clicking the “cancel” button, and it seemed to re-start with that account in mbox storage mode.)
Thanks in advance,
Roger V.