Possible regression, because did not occur on 3.4.0

Steps to reproduce the issue

On a clean multilingual 3.4.0 installation upgraded to 3.4.1-rc, go to the home page and switch to some other language than the one you initially get.

Close the browser tab, open a new one and enter in address bar the site root and press enter.

Expected result

You get to the home page with a redirect based on the previously selected language remembered with help of the cookie, but not PHP message in between.

Actual result

I got redirected in the correct way, but for a short moment a browser page with a PHP warning text and blank background was shown.

I had to stop loading before the redirect occurred to have a chance to see following PHP message:

Notice: Undefined variable: lang_code in plugins/system/languagefilter/languagefilter.php on line 297

(The root part of the URL I stripped off).

Because I do not really dare to figure out if the correct change of

$lang = $uri->getVar('lang', $lang_code);

would be just

$lang = $uri->getVar('lang');

or maybe better

$lang = $uri->getVar('lang', $this->default_lang);

or maybe more complicated, after all the discussions and multiple PRs from many people, I do not make a PR myself but prefer someone to have a look on it, e.g. @Hackwar

System information (as much as possible)

Clean multilingual 3.4.0 installation upgraded to 3.4.1-rc in a folder protected with .htaccess authentication, SEF URLs = ON, URL rewriting = ON or OFF, does not matter, all the rest regarding SEF and also the remove default language setting of the languagefilter = OFF.