Fix Issue 132, crash when navigating back without stopping drum sequencer

[misteu]

This fixes issue #132 that I reported earlier.

With this fix, the midi connection is stopped (as far as I understand the underlying method) before actually everything is de-allocated.

I could not crash the app anymore with this fix, tried navigating back and forth starting and stopping a lot of times.

[NickCulbertson]

Thanks!