is this a risky connection?
Every self transfer is risky and a 90 minute one is very risky. Sure, if all things go perfect, you have a good chance of making it. But flights can be (and often are) delayed, security lines can be long, immigration officers can be on dinner break, etc.
Assuming that you are flying with WhizzAir from Sofia: take a look at this: https://www.bbc.com/news/uk-65228868
The main problem with a self connection are the consequences of missing it: you are a no show for your second ticket and you may have to buy a brand-new same day ticket on the spot. Given the time of day, you may have to pony up for a hotel as well. That can get very expensive.