Lisbon is a quite safe big European capital.
There isn’t a significant violence crime rate, you can even walk safely at night. The safety major risk is pickpockets inside the most touristic districts and in public transportation.
In Lisbon there are “professional” pickpockets disguised as tourists, carrying cameras and maps and they work in couples or groups. They take advantage on peoples distractions while watching the beauties of the city in the most visited districts.
Take some precautions such as carrying your backpack in the front, not putting your wallet in the back pocket and always closing your purse. If you take these precautions you won’t have any problem.
If you have any problem you can always go to Tourism Police, in Restauradores square. There you’ll be seen by specialized police officers that speak several languages who will certainly help solving your problem.