Macbook has just auto-insatlled OS10.15.2 and running ACAD 2019 for Mac. 

Since the OS update has been installed, files open with x-ref paths unreadable / unallocated. The missing x-refs cannot be re-linked, as the drop down menus & buttons to do so are ghosted. The missing x-refs cannot be deleted either.

Help is much needed and will be greatly appreciated.