Call _updateFormProgress when the form
changes.
In the build() method of the _SignUpFormState class,
add a callback to the Form widget’s onChanged argument.
Add the code below marked as NEW:

    
return Form(
  onChanged: _updateFormProgress, // NEW
  child: Column(

Update the onPressed property (again).
In step 1, you modified the onPressed property for the
Sign up button to display the welcome screen.
Now, update that button to display the welcome
screen only when the form is completely filled in: