when trying to implement a custom date formatter to a field, I get the following errors:

Notice: Undefined index: custom_date_format in date_formatter_format() (line 447 of /var/www/drupalvm/drupal/web/sites/all/modules/date/date.module).
Notice: Undefined index: formatted_timezone in theme_date_display_combination() (line 139 of /var/www/drupalvm/drupal/web/sites/all/modules/date/date.theme).
Notice: Undefined index: formatted_time in theme_date_display_combination() (line 145 of /var/www/drupalvm/drupal/web/sites/all/modules/date/date.theme).
Notice: Undefined index: formatted_time in theme_date_display_combination() (line 147 of /var/www/drupalvm/drupal/web/sites/all/modules/date/date.theme).

It looks as if this is because the custom formatter field is not saving properly, as when I go to re-edit the feild is blank. Also it shows the message.

Display dates using the default format because the specified format (custom) is not defined

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roborat created an issue.