Crack on underside of handlebar
DON’T

If you are sure (or have strong suspicions) it’s a crack and not just a scratch, stop riding it. 

From your description I’m not sure what’s actually cracked (stem, fork’s steerer tube, frame’s head tube?) but replacing it is much cheaper and less painful than new teeth or sitting in a wheelchair the rest of your life.

Post a picture if you want a more accurate assessment but generally the only components on a bicycle where I’d be okay with imminent failure is anything related to gear shifting or attachments such as bottle cages or mud guards.