Flutter : Dynamic DropDownButton get error (one item with [DropdownButton]'s value)

Question

I have this dropdownbutton which get values from provider list as shown :

Consumer<OrderProvider>(builder: (context, orderProv, child) {
    print(orderProv.sizeList);
    return Container(
        width: 100,
        child: DropdownButton(
            isExpanded: true,
            value: orderProv.size,
            onChanged: (val) {
                orderProv.changeSize(val);
            },
            items: orderProv
                     .sizeList
                     .map<DropdownMenuItem<String>>((e) {
                          return DropdownMenuItem<String>(
                              value: e,
                              child: Text(e),
                          );
                      }).toList()
        ),
    );
});

At the second line print(orderProv.sizeList), when I print the dropdown menu list it prints the list correctly:

[S, M, L, XL]

But I receive the following error :

There should be exactly one item with [DropdownButton]'s value: Either zero or 2 or more [DropdownMenuItem]s were detected with the same value

Answer 1

The value provided to the DropDownButton must be one of the values in the DropdownMenuItem. Please ensure that the orderProv.size points to one of the elements in the orderProv.sizeList.