... Go back to the state you implemented on image preview where you used onchanged mehtod with setState remove controller

Then up in the state class where you set variable imageUrl call thier setState

Also check why container is having empty Url of image using logger

Use the same logic where if else is used to seperate add product with update product

Use the same way as you used in didchange find the product using find by id store it in a variable also make it nullable so incase it couldn't find any product with that ID it will be null

Now in ur if block check condition if var is null then add product else update

You can add more clear comments in your cart.dart file
As it contains many methods and variables thar are hard to understand it would be easy if you could add comments to the topic on what that particular data do