I have found a solution to this problem. What I did was that I enabled "Prevent your mac from automatically sleeping when the display is off" in Energy saver of System Preferences. Disabled "Put hard disks to sleep whenever possible". Then the most important step is to copy the photos library to your hard drive and open that library. Never quit Photos app, & while shutting down your Mac, select "Reopen windows when logging back in". And that's it. This worked for me, see if this works for you