Fixing app blowup when editing newly added instrument
Fixing app blowup when editing newly added instrument

Accidentally broken in ad81dff7a21f74034dcfeeaebb386d059dd37077

After a new track was added, clicking on the "Edit" button to edit
its instrument would cause the app to blow up. This did not occur
for tracks in the default song, because by accident default tracks
and their corresponding instruments have the same ID.

jstrait/jssynth,jstrait/jssynth