Error in listing of customers who did not make purchases Opencart

I have a problem that I can not solve at all. I'm trying to list only clients that have never made purchases, ie checking in the order table if there is no purchase associated with the client id, I'm using the query:

SELECT * FROM '" . DB_PREFIX . "customer' AS c INNER JOIN '" .
DB_PREFIX . "order' AS o ON (c.customer_id = o.customer_id) INNER JOIN
'" . DB_PREFIX . "address' AS a ON (c.address_id = a.address_id) WHERE
o.customer_id is null"

So here's my model: link

The controller: link

And the view: link

When I remove the where to only test the listing, these errors appear: link

But do not return the values, I believe that I used the where in the wrong way in this query. How can I resolve?