Flutter RenderIndexedStack object was given an infinite size during layout

I was able to reproduce your issue, I am not sure why are you building the layout this way. Your are feeding your DropDownButton as the child property for InputDecorator, which is fine. However, it is throwing this error because DropDownMenuItem is overflowing your InputDecorator. In other words, you are not only containing the DopDownButton within your InputDecorator, but also your items are trying to be contained in the same space as well. So, Flutter is confused on how to position the list of items inside the space provided by the InputDecorator.

I am really confused what kind of layout you are trying to build, why do you need a Form and an InputDecorator for?

Maybe provide a little context to your design choices so that we can help better.

I have created a simpler layout with Row and TextField widgets, it might be of help.

class MyApp extends StatefulWidget {  @override  _MyAppState createState() => new _MyAppState();}class _MyAppState extends State<MyApp> {  String _mySelection;  String _text = "Choose";   List<Map> _myJson = [    {"id": "ID 1310012", "name": "Newcommer"}, {"id": "ID 0000TEMP", "name": "Temp"}];  Widget _children() {    TextEditingController _controller = new TextEditingController(text: _text);    return  new Expanded(child: new ListView(            padding: const EdgeInsets.symmetric(horizontal: 16.0),            children: <Widget>[              new Container (        child: new Row(                  children: <Widget>[                    new Expanded(                      child: new TextField(                        controller: _controller,                      ),                    ),                    new DropdownButton(                      isDense: true,                      value: _mySelection,                      items: _myJson.map((Map map) {                        return new DropdownMenuItem<String>(                          value: map["id"],                          child: new Text(                            map["name"],                          ),                        );                      }).toList(),                      onChanged: (String newValue) {                        setState(() {                          _mySelection = newValue;                          _text = newValue;                        });                      })              ],),),              new RaisedButton(                  child: new Text('Start'), onPressed: null),            ]        ));  }  @override  Widget build(BuildContext context) {    return new Scaffold(      appBar: new AppBar(title: new Text("Test")),      body:  new Column(        mainAxisSize: MainAxisSize.max,        children: <Widget>[           _children(),        ],      ),    );  }}