Firstly, [list]the file name should not be `./lib/mian.js` but be `./lib/active-file.js`.

Secondly, in line 15 of the following code:

saveHtml () {
    // Render the content in Markdown as HTML using the Marked library
    const html = marked(this.content, { sanitize: true })

    const file = dialog.showSaveDialog(this.browserWindow, {
      title: 'Save HTML',
      // Default to the user's "documents" directory as defined by the operating system.
      defaultPath: app.getPath('documents'),
      filters: [
        { name: 'HTML Files', extensions: ['html'] }
      ]
    })

    // If the user selects cancel in the file dialog, abort the function.
    if (!this.file) { return }

    fs.writeFileSync(file, html)
  }

if (!this.file) 

should be

if (!file)

Thanks again! Fixed in the manuscript now!