In Safari (or in any modern Web browser), there's a browser console, sometimes called a JavaScript error console. In Safari, I believe you need to first enable the "Develop" drop-down menu via your Safari preferences (Safari drop-down menu --> Preferences --> Advanced --> checkbox).

Once you have a "Develop" drop-down menu in Safari, please try to reproduce the issue you're having. If you're able to reproduce the issue, on that same page, select the Develop drop-down menu --> Show Error Console. Please copy and paste any output from that error console to this bug report.

Also, can you please provide a URL to the site where you had this issue?

This happens to me at en.wp, which is the only place I've tried to use MassMessage.

I opened "Show Error Console" in Safari. I tried it several times, and the same problem happened every time. The error console was always blank. It ran a little slower, though, so I was able to see that the text (for the message body) appeared briefly in the box and then disappeared. I'm not sure if that matters.

This still happens to me every time I use MassMessage. As I only have MassMessage rights one one account (at en.wp only), I can't test it elsewhere.

I also had this happen when I tried to export a translation. I didn't try to disable the script, but it's the same sort of behavior, so I'm suspicious that it's the same cause.

Think this ancient problem is still around; see https://en.wikipedia.org/wiki/Wikipedia:Bots/Spam for a possible current example

And not just during preview!