Also here is the stack trace:
Stack trace
#0 src\addons\M2N\ContactForm\XF\Service\Contact.php(128): XF::handlePhpError(2, '[E_WARNING] Inv...', 'C:\\inetpub\\wwwr...', 128, Array)
#1 src\XF\Service\Contact.php(195): M2N\ContactForm\XF\Service\Contact->getMail()
#2...