MidiJack reuses JACK connection from AudioJack if the audio backend is JACK. The MidiJack destructor calls MidiJack::jackClient which calls m_jackAudio->jackClient(), but m_jackAudio is already deleted at the point. It causes segmentation faults.
#4384 contains a commit(abb13bd) which addresses this issue, but the PR was closed because @justnope have deleted the account.
I believe this issue is not very hard to fix.