how to validate a field if its not emplty flutter code example

Example: how to prevent users from entring null values in textfield flutter

FloatingActionButton(
            onPressed: ()async {
//                setstring(String result){
//                  widget.updatestring = result;
//                  return widget.updatestring;
//                }
                String result1 =   await Navigator.push( // string which stores the user entered value
                  context,
                  MaterialPageRoute(
                    builder: (context) => InputScreen(), //screen which has TextField
                  ));
              setState(() {
//                widget.updatestring = result1;
                TodoList(result1);
//              setstring(result1);
                addItem(result1, false); // function which adds the entered task in a list
              });
            },
            heroTag: "btn2",
            child: Icon(Icons.add, color: Color(whitecolor),), backgroundColor: Color(redcolor),),