code: @SuppressWarnings("unused")
      public static String getUserId() {
          String userId = null;
          try {
              userId = WonderPushConfiguration.getUserId();
          } catch (Exception e) {
              Log.e(TAG, "Unexpected error while getting userId", e);
          }
          return userId;
      }
docs: Gets the user id, used to identify a single identity across multiple devices,
  and to correctly identify multiple users on a single device.
  
  <p>You should not call this method before initializing the SDK.</p>
  
  @return The user id, which may be {@code null} for anonymous users.
  @see #setUserId(String)
  @see #initialize(Context)