Error message

• Notice: Undefined variable: _SESSION in CSMod\Social\Linkedin->__construct() (line 21 of /var/www/centralesupelec/sites/all/modules/site/cs_mod/classes/Social/Linkedin.php).
• Recoverable fatal error: Argument 2 passed to AppTools\Param::get() must be of the type array, null given, called in /var/www/centralesupelec/sites/all/modules/site/cs_mod/classes/Social/Linkedin.php on line 24 and defined in AppTools\Param::get() (line 51 of /var/www/centralesupelec/sites/all/modules/contrib/internal/app_tools/classes/Param.php).