src/Application/SimpleApplication.php
public function session(SessionInterface $session = null) : SessionInterface
    {
        if ($session){
            $this->session = $session;
        }
        if (!$this->session){
            $this->session = new NullSession;
        }
        return $this->session;
    }
public function session(SessionInterface $session = null) : SessionInterface
    {
        if ($session){
            $this->session = $session;
        }
        if (!$this->session){
            $this->session = new NullSession;
        }
        return $this->session;
    }
Set/get session

@param SessionInterface $session

@return SessionInterface