KeyError: 'from' when adding links

[KasiaKoz]

In a very specific scenario---when adding links to graph where edges were removed, opening up multi edge indices up for grabs---the following error appears:

        self.graph.add_edges_from(
            [
                (
>                   attribs["from"],
                    attribs["to"],
                    add_to_link_id_mapping[link]["multi_edge_idx"],
                    attribs,
                )
                for link, attribs in links_and_attributes.items()
            ]
        )
E       KeyError: 'from'

../src/genet/core.py:1345: KeyError

This only happens sometimes. We rely on networkx to generate a unique multi index, so that we can have unique edges stored on a the graph between the same pair of nodes. Sometimes the method from networkx we use returns an available index that is sandwiched between the indices we already use (it was been freed by an earlier operation on the graph), and in that case, our code fails and the data for that link disappears along the way, throwing the error seen above.