ATT testing with AL for Business Central

Hello,

I am developing a test app in AL for BC. I am stuck at one issue, when using a pagehandler method some of the fields in the test page are hidden and I am not sure how to grab those fields because once I do like ItemTrackingLines."Assign Lot No.".Invoke, If Assign Lot No is not visible on the page it will error me that the field not found.

Can anyone have any idea how to do this in AL for BC.