Leaving the fleet/ship windows open - ie - having a fleet selected, then choosing a planet to colonise crashes the game. The player (for now) must deselect any fleet, select the desired system that has a colony ship/fleet in orbit, hit colonise button next to desired planet (with NO fleet or ship selected), in order to colonise without crashing.
1 - build colony ship
2 - sent to target system
3 - colony ship/fleet arrives, select fleet
4 - select system, so both system and fleet windows are up
5 - hit colonise button next to a planet with fleet window still open, game will crash.