Opening Saved Hotbars in newer version logs error "Not a list"

The bug

When opening the Creative mode "Saved Hotbars" in a newer version (and they had not been accessed in that version before) the following error is logged:

com.mojang.datafixers.DataFixerUpper | Render thread | Not a list: 2566

Where "2566" appears to be the data version in which the Saved Hotbars were last opened.

Reproduction steps

1. Create a new launcher Installation with an older version e.g. 1.16
2. Create a new world, switch to Creative mode and store any item in the "Saved Hotbars"
3. Close the game, change the game version to the latest version and open the game again
4. Open any world
5. Click on the "Saved Hotbars" tab in Creative mode
6. Look at the log file
   An error was logged