docs: @Nullable
      public StoredClientChannel getChannel(Sha256Hash id, Sha256Hash contractHash) {
          lock.lock();
          try {
              Set<StoredClientChannel> setChannels = mapChannels.get(id);
              for (StoredClientChannel channel : setChannels) {
                  if (channel.contract.getTxId().equals(contractHash))
                      return channel;
              }
              return null;
          } finally {
              lock.unlock();
          }
      }