code: public static synchronized WonderPushChannelGroup getChannelGroup(String groupId) {
          try {
              WonderPushChannelGroup rtn = sChannelGroups.get(groupId);
              if (rtn != null) {
                  try {
                      rtn = (WonderPushChannelGroup) rtn.clone();
                  } catch (CloneNotSupportedException ex) {
                      Log.e(WonderPush.TAG, "Unexpected error while cloning gotten channel group " + rtn, ex);
                      return null;
                  }
              }
              return rtn;
          } catch (Exception ex) {
              Log.e(WonderPush.TAG, "Unexpected error while getting channel group " + groupId, ex);
              return null;
          }
      }
docs: Get a channel group.
  @param groupId The identifier of the channel group to get.
  @return The channel group, if it has previously been created using this class,
  {@code null} otherwise, in particular if an Android {@link android.app.NotificationChannelGroup}
  exists but has not been registered with this class.