I have met many solo female travelers in Indonesia. I think Indonesia is one of the ‘safer’ countries for single female travelers but ofcourse you still need to be careful.
I think Bali is probably the safer place for you to travel as are the Gili Islands. You probably have to be more careful of the tourists that visit Gili Islands than of the locals.
I have visited Sumatra many times and it is there I have met more single female travelers than any where else in Indonesia. If you visit Bukit Lawang just be careful of the Sumatran playboys with their guitars and good humour. They are mostly harmless but still need to be careful. They love to chase the ‘bule’ girls (white girls). Sort of a notch on their bed posts if you know what I mean.