code: public function run(): void
      {
          if (!$this->isAppRootSet()) {
              throw new Exception("The application root wasn't defined.");
          }
          if (!$this->isConfigFileSet()) {
              throw new Exception("The main config file wasn't defined.");
          }
          $configPath = $this->getAppRoot().$this->getConfigFile();
          if (!is_readable($configPath)) {
              throw new Exception("It's unable to load ".$configPath
                  .'as main config file.');
          }
          $config = require_once $configPath;
          if (!is_array($config)) {
              throw new Exception('The main config must be an array.');
          }
          $this->configSet = $config;
      }
docs: It loads main file of configuration and checks for validity.
  @return void
  @throws Exception It's thrown if some errors with the main configuration
  file occurred.