P: NULL photo objects resulting from folder sync

When synching a number of folders, resulting in some photos from being added, and other being removed, empty photo-objects result in the library in place of the original photos. These do not show when browsing by folder, but do show when selecting All Photographs and filtering for instance by date. They result in a blank, gray photo thumbnail, that does not have a file name or any other metadata associated (therefore the assumption these are NULL). Optimizing the catalog does not improve this situation. Issue occured on 8.1 and remains on 8.2. Working with the files now becomes very slow, as Lightroom has great problems handling the NULL objects.