A non-null String must be provided to a Text widget
You should check null safe
Text(cart_prod_qty??'default value'),
Just check for null and give a default
Text(cart_prod_qty!=null?cart_prod_qty:'default value'),You can keep it empty if you wish
Text(cart_prod_qty!=null?cart_prod_qty:''),Or else you can make text widget optional
cart_prod_qty!=null? Text(cart_prod_qty): Container()
The error itself shows what's wrong in the code, Text widget works only with string and for null they intentionally have thrown an exception. Check text.dart file implementation where they added throwing an exception.
assert( data != null, 'A non-null String must be provided to a Text widget.', ),To solve above error you have to provide some default text.
new Text(cart_prod_qty!=null?cart_prod_qty:'Default Value'),