Entities can have Computed fields which do not have any field storage definition. With Drupal 8.4 Views also supports displaying those fields.
fullcalendar_field_is_date() iterates over all fields in the view and fetches their field storage definitions. This fails as soon as any computed fields are displayed in the view.
Simply return FALSE in fullcalendar_field_is_date() for computed fields. This disallows using computed date fields as the input of the fullcalendar, but at least it avoids an error.
Closed: outdated
1.0
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
fullcalendar_field_is_date() has undergone extensive modifications during the intervening years. But I read through the function real quick and it looks like the proposed changes were implemented, albeit in a slightly different way. If I'm wrong, then feel free to re-open the issue and update the version number.