[MlMt] MM crashing

[Eric Sharakan]

I've started experiencing intermittent crashes of MailMate, since 
upgrading to r5656. I've had 3 crashes, and at least on two of them, it 
was soon after requesting a mailbox be expunged.

This sounds strangely familiar; I'm pretty sure I had a very similar 
crashing issue to this some months back, so this could be a regression 
in the latest version.

I'm running MacOS 10.14.6.

Thanks.

-Eric