Check to confirm that your Photos library is designated as the system photos library. Open Photos and do Photos/Preferences/General. Make sure that the box that says "use as system Photo library" is grayed out. Grayed out means that it already is your system photos library. If it is not grayed out, click it to gray it.
Another thing that you can try is to delete preferences. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.
-- Rich