[UE4.24] Question about GetPlayerController

[PoorMonk]

Hi all. I want to implement a function to open a new map in a Slate Widget class

void SMenuWidget::EnterGame()
{    
    UGameplayStatics::OpenLevel(UGameplayStatics::GetPlayerController(GWorld, 0)->GetWorld(), FName("NewGameMap"));
}

but, it's crashed. The reason is UGameplayStatics::GetPlayerController(GWorld, 0) return null

why is this?

Thanks for any thoughts!