src/Application/SimpleApplication.php
public function di(DiContainerInterface $di = null) : DiContainerInterface
    {
        if ($di){
            $this->di = $di;
        }
        if (!$this->di){
            $this->di = new NullDi;
        }
        return $this->di;
    }
public function di(DiContainerInterface $di = null) : DiContainerInterface
    {
        if ($di){
            $this->di = $di;
        }
        if (!$this->di){
            $this->di = new NullDi;
        }
        return $this->di;
    }
Set/get di container

@param DiContainerInterface $di

@return DiContainerInterface