Why my promise is undefined

Question

I would like use Promise in Yeoman generator. But i don't understand why my Promise is undefined when she calling in .then :

  _initDockerConfig() {
    console.log(``);
    console.log(`Init Docker config.`);
    return new Promise((resolve, reject) => {
      try {
        this.fs.copyTpl(
          this.templatePath(`docker`),
          this.destinationPath(`app/config/docker`)
        );
        resolve();
      } catch(error) {
        reject(error);
      }
    });
  }

When i call my method :

  _installSymfony() {
    return exec(`php -r "readfile(\'https://symfony.com/installer\');" > symfony`)
      .then(() => {
        console.log('');
        console.log(chalk.green('Download symfony.'));
        return exec(`php symfony new ${slugify(this.appName)}`);
      })
      .then(() => {
        return exec(`mv ./${slugify(this.appName)}/* ./`);
      })
      .then(() => {
        return exec(`rm symfony`);
      })
      .then(() => {
        return exec(`rm -rf ${slugify(this.appName)}`);
      })
      .then(() => {
        console.log(this._initDockerConfig()); // return Promise { undefined }
      });
  }

Anyone call help me ?

Thank you !