In case it helps, it was suggested I do a command-line launch with
paratext9 --debug
So I did that, and when I hit “show menu” these messages appear in the console:
(Paratext:11593): GLib-GObject-WARNING **: 10:08:13.234: invalid (NULL) pointer instance
(Paratext:11593): GLib-GObject-CRITICAL **: 10:08:13.234: g_signal_handler_disconnect: assertion ‘G_TYPE_CHECK_INSTANCE (instance)’ failed
(Paratext:11593): Gdk-CRITICAL **: 10:08:13.234: gdk_frame_clock_end_updating: assertion ‘GDK_IS_FRAME_CLOCK (frame_clock)’ failed
(Paratext:11593): GLib-GObject-WARNING **: 10:08:13.234: …/…/…/…/gobject/gsignal.c:2641: instance ‘0x7faa48c1b930’ has no handler with id ‘1912’