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[ephes]

From sentry:

            self.related_val = self.source_field.get_foreign_related_value(instance)
            if None in self.related_val:
                raise ValueError(
                    '"%r" needs to have a value for field "%s" before '
                    "this many-to-many relationship can be used."
                    % (instance, self.pk_field_names[self.source_field_name])
                )
            # Even if this relation is not to pk, we require still pk value.

[ephes]

Hmm, it's not easy to fix. Maybe wait for some Wagtail minor versions fixing this?