that becomes a question for lawyers at the appropriate time.
Put it like this:
not having a side project because you're afraid your employer will sue will get you exactly and definitely $0.
Having a successful side project will teach you a ton of useful stuff and probably earn you more than $0 even after the lawyers have finished arguing (depending on your definition of successful).
Your comment is inspiring. The question then becomes, how much of your project's worth, as a percentage, are they going to come after you for?