[BO][-] Notice messages when accessing the BO translation page

When accessing the translation part with "core translation" , you got 2 errors:
Notice: Use of undefined constant _PS_THEME_SELECTED_DIR_ - assumed '_PS_THEME_SELECTED_DIR_' in controllers\admin\AdminTranslationsController.php on line 1018

Notice: Use of undefined constant _PS_THEME_SELECTED_DIR_ - assumed '_PS_THEME_SELECTED_DIR_' in \controllers\admin\AdminTranslationsController.php on line 1026

It's because the variable $this->theme_selected is not set for that case.