TextField inside of Row causes layout exception: Unable to calculate size

you should use Flexible to use a Textfield inside a row.

new Row(
              children: <Widget>[
                new Text("hi there"),
                new Container(
                  child:new Flexible(
                        child: new TextField( ),
                            ),//flexible
                ),//container


              ],//widget
            ),//row

The solution is to wrap your Text() inside one of the following widgets: Either Expanded or Flexible. So, your code using Expanded will be like:

Expanded(
           child: TextField(
             decoration: InputDecoration(
               hintText: "Demo Text",
               hintStyle: TextStyle(fontWeight: FontWeight.w300, color: Colors.red)
              ),
           ),
        ),

(I assume you're using a Row because you want to put other widgets beside the TextField in the future.)

The Row widget wants to determine the intrinsic size of its non-flexible children so it knows how much space that it has left for the flexible ones. However, TextField doesn't have an intrinsic width; it only knows how to size itself to the full width of its parent container. Try wrapping it in a Flexible or Expanded to tell the Row that you're expecting the TextField to take up the remaining space:

      new Row(
        children: <Widget>[
          new Flexible(
            child: new TextField(
              decoration: const InputDecoration(helperText: "Enter App ID"),
              style: Theme.of(context).textTheme.body1,
            ),
          ),
        ],
      ),

You get this error because TextField expands in horizontal direction and so does the Row, so we need to constrain the width of the TextField, there are many ways of doing it.

1. Use Expanded

    Row(
     children: <Widget>[
       Expanded(child: TextField()),
       OtherWidget(),
     ],
   )
   

2. Use Flexible

   Row(
     children: <Widget>[
       Flexible(child: TextField()),
       OtherWidget(),
     ],
   )
   

3. Wrap it in Container or SizedBox and provide width

   Row(
     children: <Widget>[
       SizedBox(width: 100, child: TextField()),
       OtherWidget(),
     ],
   )