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

@param RouterInterface $router

@return RouterInterface