Hello,

This issue will be resolved in the next available version , but you can fix it on your own
file : <joomla directory>/administrator/components/com_masterleaguepro/models/multileague.php

add to the function insertUserToGroup (should be at line 346)

FROM:

Code:
public function insertUserToGroup($user,$league,$group,$points,$goal_difference){
        $query=$this->db->getQuery(true);
        $query="INSERT INTO `#__masterleaguepro_minileague_users` (user_id,minileague_id,group_id,season_points,season_goal_difference) VALUES (".$user.",".$league.",".$group.",".$points.",".$goal_difference.")";
        $this->db->setQuery($query);


        return $this->db->query();
    }
TO:


Code:
public function insertUserToGroup($user,$league,$group,$points,$goal_difference){
        if($goal_difference==""){
            $goal_difference=0;
        }
        $query=$this->db->getQuery(true);
        $query="INSERT INTO `#__masterleaguepro_minileague_users` (user_id,minileague_id,group_id,season_points,season_goal_difference) VALUES (".$user.",".$league.",".$group.",".$points.",".$goal_difference.")";
        $this->db->setQuery($query);


        return $this->db->query();
    }