List structure lost after copy-pasting

[DanielHara]

After copy-pasting, sometimes the list structure is lost, and pressing Enter does not start a new list item bullet:
I suspect this is because copy-pasting introduces paragraphs, which are not dealt with in the normalization of slate-lists.

Expected behavior:

Should strip all paragraphs insidelist-item-child elements, so that the list structure behaviour is not affected.