How to change the static mesh during the play?

Development Asset Creation
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1

I have a XML file which contains the information of every character (location,weaponID,etc).
And i write the code in the GameMode::BeginPlay() to read and parse the XML file, then spawn the character and dynamically modify the character (weapon) base on the XML.
But it doesn’t work, and I’ve already do this:

MeshComponent->SetMobility(EComponentMobility::Movable);
When i play, it shows"Access violation reading location 0x00000000000000000348"
Here is my spawn code:

  UWorld* const World = GetWorld();

	if (World)
	{
		ACharacterWithWeapon* NewActor = World->SpawnActor<ACharacterWithWeapon>(ACharacterWithWeapon::StaticClass(), SpawnLocation, FRotator(0.0f, 0.0f, 0.0f));
		UStaticMesh* weaponMesh = Cast<UStaticMesh>(StaticLoadObject(UStaticMesh::StaticClass(), NULL, *weaponMeshURL.ToString()));
		NewActor->MyWeapon->SetWeaponMesh(weaponMesh);
	}

Thank you in advance.