Foreach mysql query from an array, then a second query from that first array

I'm trying to iterate over a query from the result of another query:

    public static function getActivityFromUsers()
        {
            $database = DatabaseFactory::getFactory()->getConnection();

// FETCH USER_ID FOR THE SECOND QUERY
            $sql = "SELECT user_id, follower_since_timestamp FROM users_followers where follower_id = :user_id";
            $query = $database->prepare($sql);
            $query->execute(array(':user_id' => Session::get('user_id')));


                    if ($query->rowCount() <= 0) {
                        echo 'Nothing';
                        return false;
                    }

    $_MYSQL_DATA = $query->fetchAll();

            /* Just for my own testing and troubleshoot */
    foreach ($_MYSQL_DATA as $row) {
        echo $row->user_id . " - " . date('d/m/Y H:i:s', $row->follower_since_timestamp) ."<br/>";    
    }
            echo var_dump($_MYSQL_DATA).'<br><br>';
            /* End for my own testing and troubleshoot*/

            // USING THE USER_ID´s FROM THE FIRST QUERY TO FETCH DATA FOR OUTPUT
                foreach ($_MYSQL_DATA as $row) {
            $sql = "SELECT activity, user_id, activity_timestamp FROM user_activity WHERE user_id = :user_id";
            $query = $database->prepare($sql);
            $query->execute(array(':user_id' => $row->user_id));
            $new_data[] = $query->fetchAll();
            }


    // FOREACH TO OUTPUT DATA FROM SECOND QUERY (SHOULD BE ORDERD BY activity_timestamp)
            foreach($new_data as $key => $object){
                echo 'User_id: ' . $object->user_id.'<br>';
                echo 'What: ' . $object->activity .'<br>';
                echo 'When: ' . date('d/m/Y H:i:s', $object->activity_timestamp) .'<br>';
            }

            echo var_dump($new_data);
    }

      

        
        
        
      

    

But this gives me an error: Notice: Trying to get property of non-object in .. User_id:

etc.

var_dump $new_data[]

(from the second query that was fetching the data) gives me this: (note the two arrays at the bottom, why is this? array [2] and array [3])

array(4) { 
[0]=> array(7) { 
[0]=> object(stdClass)#7 (3) { ["activity"]=> string(15) "Wrote" ["user_id"]=> string(1) "2" ["activity_timestamp"]=> string(10) "1433704851" } 
[1]=> object(stdClass)#17 (3) { ["activity"]=> string(15) "Wrote" ["user_id"]=> string(1) "2" ["activity_timestamp"]=> string(10) "1433832032" } 
[2]=> object(stdClass)#18 (3) { ["activity"]=> string(15) "Wrote" ["user_id"]=> string(1) "2" ["activity_timestamp"]=> string(10) "1433832033" } 
[3]=> object(stdClass)#19 (3) { ["activity"]=> string(15) "Wrote" ["user_id"]=> string(1) "2" ["activity_timestamp"]=> string(10) "1433832035" } 
[4]=> object(stdClass)#20 (3) { ["activity"]=> string(7) "Follows" ["user_id"]=> string(1) "2" ["activity_timestamp"]=> string(10) "1433832150" } 
[5]=> object(stdClass)#21 (3) { ["activity"]=> string(7) "Wrote" ["user_id"]=> string(1) "2" ["activity_timestamp"]=> NULL } 
[6]=> object(stdClass)#22 (3) { ["activity"]=> string(8) "Wrote " ["user_id"]=> string(1) "2" ["activity_timestamp"]=> NULL } } 
[1]=> array(1) { 
[0]=> object(stdClass)#16 (3) { ["activity"]=> string(19) "Finally!" ["user_id"]=> string(1) "3" ["activity_timestamp"]=> string(10) "1433794873" } }
[2]=> array(0) { } 
[3]=> array(0) { } }

      

        
        
        
      

    

How to correctly determine the result from the second query?