After modified variable in blueprint but still get null at runtime

i’m new in unreal engine. I have an problem that i modified the soundbase property in blueprint but in runtime it is null, i really appreciate that someone could help me solve this problem.
here’s evidence image and my scripts


#include "CoreMinimal.h"

#include "Weapons.h"
#include "Rifle.generated.h"

class USoundBase;
class ARifle : public AWeapons
	USkeletalMesh* _rifleMesh;
	void SetRifleMesh();

	/** Location on gun mesh where projectiles should spawn. */
	UPROPERTY(VisibleDefaultsOnly, Category = Mesh)
		USceneComponent* FP_MuzzleLocation;
	void FireBullet(FRotator SpawnRotation);
	/** Gun muzzle's offset from the characters location */
	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = Gameplay)
		FVector GunOffset;

	/** Projectile class to spawn */
	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = Projectile)
		TSubclassOf<class ABullet> ProjectileClass;

	/** Sound to play each time we fire */
	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = Gameplay, meta = (AllowPrivateAccess = "true"))
		USoundBase* FireSound;


#include "Rifle.h"
#include "Components/SkeletalMeshComponent.h"
#include "Bullet.h"
#include "ShootingPlayerController.h"
#include "Kismet/GameplayStatics.h"

	_rifleMesh = NULL;
void ARifle::SetRifleMesh()
	static ConstructorHelpers::FObjectFinder<USkeletalMesh> rifle(TEXT("SkeletalMesh'/Game/PrototypeWeap/Prototype_AssaultRifle.Prototype_AssaultRifle'"));
	_rifleMesh = rifle.Object;

	FP_MuzzleLocation = CreateDefaultSubobject<USceneComponent>(TEXT("MuzzleLocation"));
	FP_MuzzleLocation->SetRelativeLocation(FVector(0.2f, 48.4f, 12.6f));
	// Default offset from the character location for projectiles to spawn
	GunOffset = FVector(100.0f, 0.0f, 10.0f);
	static ConstructorHelpers::FClassFinder<ABullet> projectileClass(TEXT("/Script/TopDownShooting.Bullet"));
	ProjectileClass = projectileClass.Class;
void ARifle::FireBullet(FRotator SpawnRotation)
	// try and fire a projectile
	if (ProjectileClass != nullptr)
		UWorld* const World = GetWorld();
		if (World != nullptr)
			// MuzzleOffset is in camera space, so transform it to world space before offsetting from the character location to find the final muzzle position
			const FVector SpawnLocation = FP_MuzzleLocation->GetComponentLocation();
			//Set Spawn Collision Handling Override
			FActorSpawnParameters ActorSpawnParams;
			ActorSpawnParams.SpawnCollisionHandlingOverride = ESpawnActorCollisionHandlingMethod::AdjustIfPossibleButDontSpawnIfColliding;

			// spawn the projectile at the muzzle
			ABullet* bullet = World->SpawnActor<ABullet>(ProjectileClass, SpawnLocation, SpawnRotation, ActorSpawnParams);

			if (bullet)
				// Set the projectile's initial trajectory.
				FVector LaunchDirection = SpawnRotation.Vector();

	// try and play the sound if specified
	if (FireSound != nullptr)
		UGameplayStatics::PlaySoundAtLocation(this, FireSound, GetActorLocation());