Cannot Spawn actor using c++

[galuodo]

HI all,
I am reading Leaning c++ by Creating Games with UE4, when i am trying spawn a bullet, it fails.

            FVector fwd = GetActorForwardVector();
	FVector nozzle = GetMesh()->GetBoneLocation("RightHand");
	nozzle += fwd * 155;// move it fwd of the monster so it doesn't 
		// collide with the monster model 
	FVector toOpponent = thing->GetActorLocation() - nozzle;
	toOpponent.Normalize();
	ABullet* bullet = GetWorld()->SpawnActor<ABullet>(
		BPBullet, nozzle, RootComponent->GetComponentRotation());

	if (bullet)
	{
		bullet->Firer = this;
		bullet->ProxSphere->AddImpulse(
			toOpponent * BulletLaunchImpulse);
		GEngine->AddOnScreenDebugMessage(0, 5.f,
	}
	else
	{
		GEngine->AddOnScreenDebugMessage(0, 5.f,
			FColor::Yellow, "monster: no bullet actor could be spawned. is the bullet overlapping something ? ");

}

the spawn return null all the time. then I clear the static mesh on bullet, it can spawn correctly.
Is there something wrong?

[andrew-r-lukasik]

What is BPBullet exactly?

[grouptout]

CPP:

if (IsValid(BPBullet))
{
	GetWorld()->SpawnActor<ABullet>(BPBullet, GetActorTransform());
}

H:

UPROPERTY(EditAnywhere, BlueprintReadWrite)
	TSubclassOf<ABullet> BPBullet;

BP:

[galuodo]

it defines as

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = MonsterProperties)
UClass * BPBullet;

A blue print class of the bullet: