Object Reference Not Set ... Debugging Stored Procedure in Visual Studio

I'm working on updating a stored procedure that is called to update a user. This stored procedure needs to also create a description of what was updated for auditing purposes. However when debugging in Visual Studio I get!

All of my input values are valid, I leave none null, and my update_* procedures without the action_taken value are working. I'm seemingly writing something incorrectly, but I haven't figured out what.

CREATE DEFINER=`root`@`localhost` PROCEDURE `update_user`(in requestor_id BIGINT(20),in user_id BIGINT(20),in department_id BIGINT(20),in location_id BIGINT(20),in shift_id BIGINT(20),in first_name VARCHAR(45),in last_name VARCHAR(45),in email VARCHAR(45),in phone VARCHAR(45),in security_stamp LONGTEXT)BEGIN    DECLARE queried_username VARCHAR(45);    DECLARE action_taken VARCHAR(255);    SET action_taken = '';    SELECT        users.username    INTO        queried_username    FROM users     WHERE users.id=user_id    LIMIT 1;    IF (queried_username IS NULL) THEN    BEGIN    END;IF (department_id != users.department_id) THEN    SET action_taken = CONCAT(action_taken, '[Department Updated]');END IF;IF (location_id != users.location_id) THEN    SET action_taken = CONCAT(action_taken, '[Location Updated]');END IF;IF (shift_id != users.shift_id) THEN    SET action_taken = CONCAT(action_taken, '[Shift Updated]');END IF;IF (first_name != users.first_name) THEN    SET action_taken = CONCAT(action_taken, '[First Name Updated]');END IF;IF (last_name != users.last_name) THEN    SET action_taken = CONCAT(action_taken, '[Last Name Updated]');END IF;IF (email != users.email) THEN    SET action_taken = CONCAT(action_taken, '[Email Updated]');END IF;IF (phone != users.phone) THEN    SET action_taken = CONCAT(action_taken, '[Phone Updated]');END IF;    BEGIN        UPDATE users SET             users.department_id=department_id,             users.location_id=location_id,             users.shift_id=shift_id,            users.first_name=first_name,            users.last_name=last_name,            users.email=email,            users.phone=phone,            users.modified=NOW(),            users.security_stamp=security_stamp        WHERE users.id=user_id;        CALL update_audit_log(            requestor_id,             user_id,            null,            department_id,             location_id,             shift_id,             null,"User Administration",             CONCAT('Modified user (id="',                user_id,'", username="',                queried_username,'") | Action= ',                action_taken            )        );    END;    END IF;END

Any help is very much appreciated!