Return the NettingChannelState if it exists, None otherwise.

def get_channelstate_by_canonical_identifier(
        chain_state: ChainState,
        canonical_identifier: CanonicalIdentifier,
) -> Optional[NettingChannelState]:
    """ Return the NettingChannelState if it exists, None otherwise. """
    token_network = get_token_network_by_identifier(
        chain_state,
        TokenNetworkID(canonical_identifier.token_network_address),
    )

    channel_state = None
    if token_network:
        channel_state = token_network.channelidentifiers_to_channels.get(
            canonical_identifier.channel_identifier,
        )

    return channel_state