There is an error in \a3\modules_f_heli\misc\functions\modulespawnai\main.sqf where a wrong variable is referenced.
line 223:
//update the global pool of groups
if !(_group isEqualTo _groupsPrev) then
{_group should be _groups. Compare groups just spawned against the current reference of groups previously spawned held on the module.
If the manpower setting in the spawnAI module is low enough that nothing can be spawned, _group will be nil leading to script errors being shown.
Use the attached test mission.
The spawnAI module is set to only spawn 1 infantry "max Soldiers per group" and its manpower cap to 0.2 ( A blufor B_soldier_F's manpower rating is 0.4, ) so nothing can be spawned causing the error.
Change the manpower cap to > 0.4 and everything will work ok.
This is set up to specifically show the error but this can happen at any time especially when using armoured groups where their manpower rating is a lot larger ( e.g B_MBT_01_cannon_F is greater than 23 for the tank and its spawned crew ).