I managed to solve this problem in a very simple way that doesn’t require much technical knowledge, and it only takes a few minutes!
Steps:
- Install and open Notepad++.
- Click Search → Find in Files….
- Search for “NoneNone” (without quotation marks). Make sure that “In all sub-folders” is ticked. Choose your Content directory as directory, and/or the folder where your Blueprints are placed.
- The search procedure should find at least one Blueprint (.uasset) file with a match. Remove all object variables (including e.g., actor references, arrays, maps) in the corresponding Blueprint(s) WITHIN the editor (do not try to change the binary file, it will break things), and replace the variables with fresh ones (they can be of the exact same type). Save your Blueprint.
- Try packaging/building/cooking again. The problem should have been solved. If the problem still persists, try doing the same for all local variables in the matching Blueprint’s different functions.