Clearing text field won't erase all data

Hi I'm trying to convert different units of weight. when I enter an amount and clear it, the 2nd text field would still show the amount other than showing "0". How can I get through this issue. There are 2 pictures below.

[

I want to get the 2000 to remove after deleting too. my code is below.

class Weight extends StatefulWidget {

  @override
  _WeightState createState() => _WeightState();
}

class _WeightState extends State<Weight> {
  String _value1;
  String _value2;
  TextEditingController _controller;
  TextEditingController _controller2;
  static final Map<String,double> changes= {
    "gram-kilogram": 0.001,
    "kilogram-gram": 1000,
    "kilogram-kilogram": 1,
    "gram-gram": 1,
  };
  initState(){
    super.initState();
    _controller=TextEditingController();
    _controller2=TextEditingController();
  }
  dispose(){
    super.dispose();
    _controller.dispose();
    _controller2.dispose();
  }

There is more code here I deleted cause too much code.

            new TextField(
              controller: _controller,
              onChanged: (text)=>{
                _controller2.text=(double.parse(text)*changes[_value1+"-"+_value2]).toString(),
              },
              decoration: InputDecoration(
                border: OutlineInputBorder(),
                labelText: ("Enter amount in $_value1")
              ),
              keyboardType: TextInputType.number,
              style: new TextStyle(
                fontFamily: "Poppins",
              ),
            ),
            new Padding(padding: EdgeInsets.only(top: 50.0)),
            new DropdownButton(
              items: [
                DropdownMenuItem(child: Text("Kilogram"),
                  value: "kilogram",
                  onTap: (){
                  },),
                DropdownMenuItem(child: Text("gram"),
                  value: "gram",
                  onTap: (){
                  },),
              ],
              onChanged: (String value){
                setState((){

                  _value2 = value;
                  print("$value");
                });
              },
              hint: Text('Select Item'),
              value: _value2,
            ),
            new TextField(
              controller: _controller2,
              onChanged: (text)=>{
                _controller.text=(double.parse(text)*changes[_value2+"-"+_value1]).toString(),
              },
              decoration: InputDecoration(
                  border: OutlineInputBorder(),
                  labelText: ("Enter amount in $_value2")
              ),),],),),);}}