unit group still invalid during selection code

[kyranf]

after v0.4.17 we still get some instances of the unit group invalid while checking for squad validity during selection checks

https://mods.factorio.com/mod/robotarmy/discussion/6374a63678ccfa47fd725a94

模组 机器人军队 (0.4.17) 引发了无法恢复的错误。
请向模组作者报告此错误。

Error while running event robotarmy::on_player_selected_area (ID 51)
LuaUnitGroup API call when LuaUnitGroup was invalid.
stack traceback:
[C]: in function 'index'
__robotarmy/robolib/Squad.lua:640: in function 'validateSquadIntegrity'
robotarmy/robolib/Squad.lua:289: in function 'getClosestSquadToPos'
robotarmy/robolib/eventhandlers.lua:119: in function 'reportSelectedUnits'
robotarmy/control.lua:75: in function <robotarmy/control.lua:74>