Your photo library is either in use by another application or has become unreadable. I the error message from iPhotos after the so called upgrade to Photos which left me with a blank screen

Photos upgrade a disaster. No images appear in Photos. Able to re-establish all images in iPhoto only to have the app crash after uploading new photos from iPhone. Impossible to reload photo library as it needs 161 GB of space. The non functioning Photos library is 107 GB and the original iPhotos library from which it was created is 118 GB. I only have 146 GB available on this 1 TB drive but I am reluctant to trash Photos library in case iPhotos is truly damaged.