To reproduce:
1. Create a new tab.
2. Attach a device to sync music to.
3. In that new tab, select the portable device in the left bar.
4. Disconnect the portable device from your computer.
5. Quit MusicBee.
6. Relaunch MusicBee.
7. Notice that there appears to be only one tab.
8. Press ctrl+tab.
9. The tab that you had been looking at the portable device in becomes focused, even though it's still not visible in the tab bar.