If you deactivate dokan, is this notice go away? If yes please let dokan developer know this. You can give them this link so that they understand where the issue is. https://github.com/shamim2883/front-end-pm/blob/75eb5f4b1ca72910f499e4733e376f20e8896577/functions.php#L1584
For temporary solution you can replace $args[0] with @$args[0] in functions.php on line 1584