Actually its common for many if not all employers to ban outside work with out approval from your manager.

Which is bullshit and absolutely none of their business.

And also not very enforceable in some areas. I got legal advice on this from my own attorney some 20 years ago. I had created a piece of software on the side while working under a contract that said something similar. The software took off and I was worried about it. He advised me that worst case would be paying the company the money I earned while under that contract, but even that was a stretch.

In addition to being bullshit it is also a hint you should be moving on.

In the minority of cases where this is true, just handle it differently.