crash when change mode if no PCGFoliageMananger

[hiram3512]

TArray<FLinearColor> APCGFoliageManager::GetBiomePreviewColor()
{
	FRandomStream Random(0);
	TArray<FLinearColor> Result;
	for (int i = 0; i < Biomes.Num(); i++)
	{
		const uint8 Hue = (uint8)(Random.FRand() * 255.f);
		Result.Add(FLinearColor::MakeFromHSV8(Hue, 255, 255));
	}
	return Result;
}

if have no APCGFoliageManager in level, change mode will call this method, and crash