Night bus from London to Heathrow - Is it SAFE?

Hi! I'm spending 2 nights in London in July. My returning flight departures at 06:20am from Heathrow. So, I must arrive around 04:20am there. The only way to get to Heathrow at this time is taking N9 night bus. But I am concerned about SAFETY... Is it safe for a tourist with luggage to take this night bus? Thank you!