Magento 2 Add Image Upload in Configuration issue “The base directory to upload file is not specified”

I am new in MAgento2, and I installed custom module of Magento 2.3 to add image file upload configuration in system.xml.
But i encountred this isuue when i try to save the config :

system.xml

< ?xml version="1.0"?>

    
        
            Mageplaza
        
        

            separator-top
            Hello World
            mageplaza
            Mageplaza_HelloWorld::helloworld_config
            
                General Configuration

                
                    Image Upload
                    
                    MagentoConfigModelConfigBackendImage
                    don/post
                    
                    < ![CDATA[Allowed file types: jpg, jpeg, gif, png, svg]]>
                

            
        
    

my backend_model Image.php :

< ?php
namespace MageplazaHelloWorldModelConfigBackend;

class Image extends MagentoConfigModelConfigBackendImage
{
    /**
     * The tail part of directory path for uploading
     */
    const UPLOAD_DIR = 'don/post';

    /**
     * Upload max file size in kilobytes
     *
     * @var int
     */
    protected $_maxFileSize = 2048;

    /**
     * Return path to directory for upload file
     *
     * @return string
     * @throw MagentoFrameworkExceptionLocalizedException
     */
    protected function _getUploadDir()
    {
        return $this->_mediaDirectory->getAbsolutePath($this->_appendScopeInfo(self::UPLOAD_DIR));
    }

    /**
     * Makes a decision about whether to add info about the scope.
     *
     * @return boolean
     */
    protected function _addWhetherScopeInfo()
    {
        return true;
    }

    /**
     * Getter for allowed extensions of uploaded files.
     *
     * @return string[]
     */
    protected function _getAllowedExtensions()
    {
        return ['jpg', 'jpeg', 'gif', 'png', 'svg'];
    }

    /**
     * @return string|null
     */
    protected function getTmpFileName()
    {
        $tmpName = null;
        if (isset($_FILES['groups'])) {
            $tmpName = $_FILES['groups']['tmp_name'][$this->getGroupId()]['fields'][$this->getField()]['value'];
        } else {
            $tmpName = is_array($this->getValue()) ? $this->getValue()['tmp_name'] : null;
        }
        return $tmpName;
    }

    /**
     * Save uploaded file before saving config value
     *
     * Save changes and delete file if "delete" option passed
     *
     * @return $this
     */
    public function beforeSave()
    {
        $value = $this->getValue();
        $deleteFlag = is_array($value) && !empty($value['delete']);
        $fileTmpName = $this->getTmpFileName();

        if ($this->getOldValue() && ($fileTmpName || $deleteFlag)) {
            $this->_mediaDirectory->delete(self::UPLOAD_DIR . '/' . $this->getOldValue());
        }
        return parent::beforeSave();
    }
}

I created also the folder pub/media/don/post/default

i did not succeed in finding a solution of this issue

Thanks in advance