code: @Nonnull
    public PhotonSessionStatePerApp state (@Nonnull @Nonempty final String sAppID)
    {
      ValueEnforcer.notEmpty (sAppID, "AppID");
      return m_aStateMap.computeIfAbsent (sAppID, k -> new PhotonSessionStatePerApp ());
    }
docs: Get or create a new state for the provided app ID.
  
  @param sAppID
  The app ID to get the state for. May neither be <code>null</code>
  nor empty.
  @return Never <code>null</code>.