Does the login process still work? That's the most important question and, if so, you can simply hide that notice by going to the Global Configuration area and setting Error Reporting to 'None'. A PHP Notice is not really a problem, it's just letting you know something isn't perfect and may need investigation.
The other suggestion would be to contact Mautic. It sounds like their plugin is looking for *any* response from LinkedIn when there are ways that it could be looking for responses that are only targeted at Mautic and not ones for JFBConnect.
I hope that helps,
Alex
Conflict with Mautic plugin
