I was in Bali last week, I stayed for two weeks and in this two weeks I experienced two earthquakes. One of them was in the nearby island of Lombok, but it was too strong and it was felt in Bali, the other one was centered 60km from Bali but it wasn't that strong (5.6) but I was in the 4th floor and it was scary. However, nothing happened and no injuries at all, many of the people who were asleep didn't even wake up.
Bali is located inside the ring of fire, which is an area where many earthquakes and volcano eruptions occur. That's a fact and no one can do anything about it.
How often do earthquakes happen? when will they happen? how strong will they be? Unfortunately, these questions are not answerable. But remember, mankind lived there for thousands of years and managed. Most buildings will stand most of the earthquakes. Most deaths occur from the destruction of weak buildings that are usually found in slums and in small villages where houses are built in the cheapest possible way with no support whatsoever. Checking the photos of the destroyed buildings in Lombok after the last serious earthquake revealed no large buildings were destroyed. I do not think that you would stay in one of those cheap houses.
The real problem from earthquakes are the tsunamis that follow, most hotels have a Tsunami Gathering Point in higher floors where all guests will be directed to go to in case there was a tsunami alert. Indonesian government issue tsunami alerts after earthquakes if they are possible, if you ever felt an earthquake just type "tsunami alert" in google and you should be aware if there's one.
Regarding volcanoes, Mt. Agung could erupt in Indonesia and affect the aviation around the world, just like what happened with Eyjafjallajökull eruption in 2010. So, your flight might be delayed due to an eruption in a different part of the world due to wind directions, while you could be close to the volcano and your flight takes off on time. I wouldn't worry about this.