- Fixed UI app deadlocks which can happen when the user quickly pushes "Play" button in the MIDI player.
  Although, this kind of deadlock is possible with any MidiDriver, the probability of it is very low with all the other drivers as the waiting is performed upon application shutdown only. So, even if it's ever happened, it shouldn't harm too much.
  Test MIDI driver is a bit special. The deadlock is more difficult to reproduce as the test driver can be started via menu only and there is sufficient delay. But it might become more probable in case of UI change or making some other usage of the test driver.