Motion-list > multi-select: Prevent Error in snackbar

[Elblinator]

Current behaviour:
If a several motion are added to the agenda and at least one motion is already in the agenda then a cryptic error is thrown: Error: Update of agenda_item/3395: You try to set following required fields to an empty value: ['content_object_id']

Reproduction:
in a meeting:

1. have at least one topic in agenda
2. create two motions, put one of them in the agenda
3. navigate to motion-list
4. activate multi-select and select the two motions
5. move them into the agenda (select the topic as parent)
6. error is thrown

Wanted behaviour:
If one of the motions is already in the agenda, then this motion should be skipped and the rest should be continued