Q:

A non-null String must be provided to a Text widget

I'm trying to add the option for the quantity to be adjusted but I get an error saying "A non-null String must be provided to a Text widget"
How do I provide this, to this code? 
 trailing: Container(
        height: 60,
        width: 60,
        padding: EdgeInsets.only(left: 10),
        child: Column(
          children: <Widget>[
            new GestureDetector(child: Icon(Icons.arrow_drop_up), onTap: () {}),
            new Text(cart_prod_qty),
            new GestureDetector(child: Icon(Icons.arrow_drop_down), onTap: () {})
          ],
        ),

A:

You should check null safe
Text(cart_prod_qty??'default value'),

A:

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'),

A:

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()

