Fixed undefined notice in the dialog

      ];

      foreach ($attributes as $plugin) {

        $form['linkit_attributes'][$plugin->getHtmlName()] = $plugin->buildFormElement($input[$plugin->getHtmlName()]);

        $form['linkit_attributes'][$plugin->getHtmlName()] += [

        $plugin_name = $plugin->getHtmlName();

        $default_value = isset($input[$plugin_name]) ? $input[$plugin_name] : '';

        $form['linkit_attributes'][$plugin_name] = $plugin->buildFormElement($default_value);

        $form['linkit_attributes'][$plugin_name] += [

          '#parents' => [

            'attributes', $plugin->getHtmlName(),

            'attributes', $plugin_name,

          ],

        ];

      }