Severe instability issues with MuseScore 3.0 on Ubuntu 18.04

So I recently noticed the MuseScore 3.0 development version available for download, and decided to test the new version--knowing that it would likely have a few bugs. (Development info: OS: Ubuntu 18.04.1 LTS, Arch.: x86_64, MuseScore version (64-bit): 3.0.0., revision: 0526317)

My issue is that MuseScore 3.0 Dev for me is extremely susceptible to random "hanging", forcing me to terminate the program and reopen it. (I usually can't use it for more than 30 minutes without it "hanging.") I did not have problems like this with MuseScore 2.3.2 (revision 4592407), which is still installed on my laptop. Just FYI: The new (to me) "Bug Report" and "Leave Feedback" buttons do not work (nor do the corresponding menu entries in the "Help" menu).
As a programmer myself, I know how critically important it is to pinpoint a specific issue that can reliably be reproduced...but the problem here is that a particular action might crash MuseScore 3.0 two or three times in a row--and then work perfectly afterwards.

It seems to be tied most closely to MIDI output, as the most frequent scenario is when I'm entering notes in MuseScore (with my laptop's keyboard, not a MIDI input device). Sometimes (like today) it "hung" when I tried entering the first note. So I enter a note, it plays the note...then immediately hangs, leaving the note playing. (When it "hangs", some menus and buttons may respond to the mouse for a little bit, but before long the only button I can hammer on is the "X" to close the window.) Terminate and restart the program, try to enter a note--and it instantly hangs again (leaving the note playing). Third time was the charm, as I was able to enter notes for a bit. (I have had it "hang" when entering notes well over a dozen times in the two or so days I've tried to use it.)

It also seems to be tied to the workspace. I have the settings configured to "restore last session on restart." When I reopen MuseScore 3.0 after terminating it, it asks me, "Last session stopped unexpectedly. Restore last session?" Even if I click the "no" button, it reopens the last session--and more often than not will immediately "hang" when I try to enter a note. (Did that two times in a row this morning...then the third time, it didn't "hang" when I tried to enter the notes. Goes without saying that I lose all unsaved work when I have to terminate it.)

For debug purposes (to rule in or out my Ubuntu installation), I tried opening the MuseScore file in v2.3.2--needless to say, it didn't work. (May try an XML export-import to 2.3.2.)
I have used MuseScore 2.3.2 on Ubuntu 16.04 and 18.04 for a number of months without experiencing this issue. (Not sure if this means anything, but on Ubuntu 16.04 I used the default Unity desktop, while on 18.04 I tried Cinnamon for awhile, then just last week switched to KDE.)

Anything else I could try to help pinpoint the issue?

EDIT: Seems that the reason it fails twice, then works the third time...is because when restoring a crashed workspace, the first time it reloads the session very close to as you had it (minus a change or two)--whereas the second time you reload the session, it loses all changes back to the last time the score was saved.
I'm also having a 100% reproducible issue with MuseScore "vanishing" [NOT hanging] when changing time signatures, which appears to have been partially addressed in another bug report. Seems to have to do with the content of my particular score, which I attached below.
Steps to reproduce the "vanish": Open the attached score. Scroll to measure 27, and set the time signature to 2/4. Select measure 28, and set it to 4/4. Dollars to donuts if MuseScore doesn't vanish from your screen.