Note for buster-backports:

Polyphone is a bit crashy especially when the JACK dæmon (jackd)
does not already run before Polyphone is started. This is mostly
caused by the bug https://bugs.debian.org/944509 in a library in
the version included with buster. https://bugs.debian.org/982669
tracks the attempt to provide a fix.

Until the fix is included in a buster update, please start JACK,
for example using qjackctl, before running Polyphone. There will
still be a crash when exiting; at that point however all data is
already saved, so that crash is harmless.