Don't tell your employer. Don't incorporate until after you've quit your job. If you want to launch before quitting your job, consider using a merchant of record (e.g. Paddle) so you don't even need to incorporate. I'm assuming you're not competing with your employer, nor are you building your startup on your employer's time or with your employer's resources, in which case there's nothing wrong with this approach eth…
Don't tell your employer. This may make it less likely that your employer will take you to court, but if you do this and your employer does choose to pursue a case against you after you leave the court will see it as intentionally deceptive and it definitely won't help your argument.
Above reasoning is in line with why companies don't search for existing patents when developing new technologies. If they search for prior work and are found guilty of infringement, that's considered willful infringement, in which case damages are tripled.