PostLab will automatically clear your recent libraries list in Final Cut Pro. So every time you open Final Cut Pro you’ll see something like this:
An empty list. This is done for safety reasons. When using PostLab you must open libraries through the application. This way you get the latest version and other people aren’t able to work on the same library. Not clearing the recent libraries would allow you to open a library that might have changed in the mean time by someone else.
So if your working with PostLab open your libraries with PostLab and your fine!