Endless validating usually means that there is a corrupt project in the iMovie library. To test, open iMovie in a new blank library. To do that open iMovie while holding down the Option key. You will get a box that will give you the option to open iMovie in a new library. If it opens in the new blank library (with no projects in it) that would indicate that there is a corrupt project in your old library that is hanging up the opening process.
For a solution, you might take a look at this thread and see if there is anything in there that helps you:
https://discussions.apple.com/thread/250312276?answerId=250610699022&page=1
-- Rich