You need to call it this way:
BehaviorComp = CreateDefaultSubobject<UBehaviorTreeComponent>(TEXT("BehaviorComp"));
Also, since you are overriding the Possess function you need to call its Super implementation first:
void AAI_Controller::Possess(APawn * InPawn)
{
Super::Possess(InPawn);
}
Otherwise, your possessing won’t work properly. (Credits to Daxter154 for the catch)