No value in a variable if col. in the list is hidden

I wrote some code in the Item list to show the items where Inventory is < 1 in red.

This works perfect but when the Inventory field is hidden, all lines show up in red. It seems that when a column is hidden, this field of the record does not contain any value anymore, in this case making all the lines show up in red.

How can I make this works even when the inventory field is hidden?