musescore 2.0 segfault in mscore/seq.cpp:643

On git commit b1d7b217ba1a6ebe3c845f4f0b4b7226e84740df

I've managed to get musescore into a state where it segfaults every time I run it. The splash screen appears but isn't rendered, so it's fairly early on.. I'm trying to use JACK audio (mscore -a jack), and had a look at the problem in GDB:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffe3be6700 (LWP 11425)]
Ms::Seq::process (this=0x7fffe56ff010, n=n@entry=1024, 
    buffer=buffer@entry=0x7fffe3be3dc0)
    at /home/sqweek/tmp/src/MuseScore/mscore/seq.cpp:643
643	                  if (cs->playMode() == PLAYMODE_SYNTHESIZER) {
(gdb) print cs
$1 = (Ms::Score *) 0x0

If I'm reading the code correctly, this is within a "if (state == TRANSPORT_PLAY) {" block, so somehow the transport state has been persisted in PLAY mode and it's trying to play before a score has been loaded?

I'm not exactly sure how I got here, but I do remember killing a previous instance of mscore with SIGINT (ctrl-c) when it started eating memory and causing my machine to swap. I think I had just turned off jack MIDI (which I used to use in 1.2) in an attempt to try out the internal synth with Jack output.