BoxConstraints forces an infinite width

I am getting an error when I add a row in a column. I am getting the following error:

I/flutter ( 6449): ══╡ EXCEPTION CAUGHT BY RENDERING LIBRARY ╞═════════════════════════════════════════════════════════
I/flutter ( 6449): The following assertion was thrown during performLayout():
I/flutter ( 6449): BoxConstraints forces an infinite width.
I/flutter ( 6449): These invalid constraints were provided to RenderAnimatedOpacity's layout() function 

Also for reference here is my code:

return new Scaffold(
      backgroundColor: whiteColor,
      body: new Column(
        children: <Widget>[
          imgHeader,
          lblSignUp,
          txtEmail,
          Row(
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[
              txtFirstName,
              txtLastName
            ],
          ),
        ],
      ),
    );

If you are using TextField Inside a Row then you need to use Flexible or Expanded.

More details are given here in this answer.

https://stackoverflow.com/a/45990477/4652688

Reason for the error:

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()),
       // more widgets
     ],
   )
   

2. Use Flexible

   Row(
     children: <Widget>[
       Flexible(child: TextField()),
       // more widgets
     ],
   )
   

3. Wrap it in Container or SizedBox and provide width

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

This happens only when you are trying to define an infinite width and height for a Row, Column or Container that is a parent of the TextField that wants to use all the space.

To solve this, you should wrap your TextField with Flexible or Expanded.

Do this to solve the issues above and beyond.

 Expanded(
   child: txtFirstName,
 ),

 Flexible(
   child: txtLastName,
 ),

Full example

 Column(
    children: <Widget>[
      imgHeader,
      lblSignUp,
      txtEmail,
      Row(
        mainAxisAlignment: MainAxisAlignment.start,
        children: <Widget>[
          Expanded(
            child: txtFirstName,
          ),
          Flexible(
            child: txtLastName,
          ),
        ],
      ),
    ],
  )