Return the NettingChannelState if it exists, None otherwise.

def get_channelstate_by_token_network_and_partner(
        chain_state: ChainState,
        token_network_id: TokenNetworkID,
        partner_address: Address,
) -> Optional[NettingChannelState]:
    """ Return the NettingChannelState if it exists, None otherwise. """
    token_network = get_token_network_by_identifier(
        chain_state,
        token_network_id,
    )

    channel_state = None
    if token_network:
        channels = [
            token_network.channelidentifiers_to_channels[channel_id]
            for channel_id in token_network.partneraddresses_to_channelidentifiers[partner_address]
        ]
        states = filter_channels_by_status(
            channels,
            [CHANNEL_STATE_UNUSABLE],
        )
        if states:
            channel_state = states[-1]

    return channel_state