How to do it...

1. In the DelegateListener class, add the following overridden function declaration:

UFUNCTION() 
virtual void EndPlay(const EEndPlayReason::Type EndPlayReason) override;

2. Implement the function like this:

void ADelegateListener::EndPlay(const EEndPlayReason::Type EndPlayReason)
{
    Super::EndPlay(EndPlayReason);
    UWorld* TheWorld = GetWorld();

    if (TheWorld != nullptr)
    {
        AGameModeBase* GameMode = 
                               UGameplayStatics::GetGameMode(TheWorld);

        AChapter_05GameModeBase * MyGameMode = 
                               Cast<AChapter_05GameModeBase>(GameMode);

        if (MyGameMode != nullptr)
        {
            MyGameMode->MyStandardDelegate.Unbind();
        }
    }
}