Zip, zero, nada — just another way to say null, that pesky word that may be causing problems on your category pages. If your pages are returning an error message, check to see if the Custom SQL Filter field for the category contains the word “NULL.” This situation is most common when Category data is edited in an external program and imported into your store. Who knew?
Never has so much nothing caused so much to be concerned about! Here’s how to fix it:
Go to Inventory > Categories and click Search.
Go to Advanced Settings > Misc and type "NULL" into the Custom SQL Filter field. Click Search.
All categories with NULL in the Custom SQL Filter field, will be listed in the search results. Click the three-dot actions menu icon and select Customize Columns.
In the Customize Columns dialog box, scroll and check the Custom SQL Filter box and click Apply Changes.
The search results table will now have a Custom SQL Filter column. Hover over the Custom SQL Filter column header and click Edit to open the column for editing.
Delete "NULL" from every category (or replace it with a valid SQL clause) and click Save.
When you are finished, hover over the column header again and click Disable Edit to close the column from editing.
This should resolve the error on your storefront categories.
Removing a lot of nothing from your category pages may sound funny, but it can eliminate those pesky error messages.