`sectors.size - 1` is `-1` when there's no sectors on planet.
relevant crash log:
```
[E] java.lang.ArrayIndexOutOfBoundsException: Index -1 out of bounds for length 16
at arc.struct.Seq.get(Seq.java:451)
at mindustry.type.Planet.getLastSector(Planet.java:114)
at mindustry.ui.dialogs.PlanetDialog.show(PlanetDialog.java:185)
...
at mindustry.desktop.DesktopLauncher.main(DesktopLauncher.java:36)
```