On getValidChoices, it tries to load all the storage definitions from config fields but base fields can also define allowed values and then the definition will be null, getFieldStorageDefinitions needs to be used instead (from the entity field manager) that returns both base fields and config fields.
Patch attached for 4.x, I think the same file content is present on 1.x so could be applied in both branches.
I suspect that this will fix #2985361: "The value you selected is not a valid choice" error is thrown when a workflow field is present → too
Needs review
4.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Thanks for the patch, It didn't work for me, but I assume it is for an older version of select_or_other, in my case, I have a custom entity with select list property, which gets ignored in mustBeValidatedByCore() because it doesn't want to deal with select list base fields!
I guess this would be a re-roll.