Play Panel (F11)

I don't see it in the issue tracker, so please go ahead and enter it there

Stack trace (for master):
1 Ms::Seq::curTempo seq.cpp 1556 0x4f7c8f
2 Ms::PlayPanel::relTempoChanged playpanel.cpp 100 0x4e7783
3 Ms::PlayPanel::relTempoChanged playpanel.cpp 112 0x4e7806
4 Ms::PlayPanel::qt_static_metacall moc_playpanel.cpp 128 0x6e60de
5 ZN11QMetaObject8activateEP7QObjectiiPPv 0x68c72fb2
6 ZNK8QSpinBox5fixupER7QString 0x20753bc5
7 ?? 0x7f000003
8 ??

double Seq::curTempo() const
      {
      return cs->tempomap()->tempo(playPos->first);
      }

cs doesn't exist anymore after having closed all scores

Possible fix (tested, does fix the crash, and I don't see any bad side effects):

double Seq::curTempo() const
      {
      return cs? cs->tempomap()->tempo(playPos->first) : 0.0;
      }