@gardian206 beat me to it! Indeed, TestPickup can be null if the cast fails and that causes a crash.
@Star.Zhou You could also consider refining your overlap check to filter for your AACollectActor class. Something like:
const TArray<TEnumAsByte<EObjectTypeQuery>> ObjectTypes;
const TArray<AActor*> ActorsToIgnore;
TArray<AActor*> OutActors;
UKismetSystemLibrary::SphereOverlapActors(GetWorld(), GetActorLocation(), 100.f, ObjectTypes, AACollectActor::StaticClass(), ActorsToIgnore, OutActors);