Adding track number n >= 9 or 10 results in clone track of nth-1 track

[PeterGhimself]

Steps to reproduce

1. Open tuxguitar.
2. Click Track -> Add Track (or use Ctrl + Shift + Insert shortcut) 7 times, now you should have 8 (default) Steel string instrument tracks.
3. Change the 8th track to another instrument: click Track -> Properties and select any other instrument from the drop down. Click OK.
4. Add another track. Notice it is not the default "Steel String Guitar", but whatever you set the 9th track to.
5. Try changing any of the properties of these clones, the changes will be set for each of them (confirming cloned track behavior).

Notable fishiness 🐟

On a blank file with one measure I was able to consistently get this behavior reproduced after starting on the 9th track, however when in the middle of working on a more busy file this issue only occured on the 10th track.

Testing environment

Windows 10, using 32-bit Java 8.