Testing the mod in creative. First game I got the same error very early without launching a single rocket. Second game, quite a bit later after many burned aliens.
I encountered the same or a similar error today with the current mod version, 1.5.2.
Error while running event Orbital Ion Cannon::on_biter_base_built (ID 55) Orbital Ion Cannon/control.lua:196: bad argument #1 to 'pairs' (table expected, got nil)