Locate the build() method for the
_SignUpFormState class. This is the part of the code
that builds the SignUp button.
Notice how the button is defined:
It’s a TextButton with a blue background,
white text that says Sign up and, when pressed,
does nothing.

Update the onPressed property.
Change the onPressed property to call the (non-existent)
method that will display the welcome screen.

    Change onPressed: null to the following:

    
onPressed: _showWelcomeScreen,

Add the _showWelcomeScreen method.
Fix the error reported by the analyzer that _showWelcomeScreen
is not defined. Directly above the build() method,
add the following function:

    
void _showWelcomeScreen() {
  Navigator.of(context).pushNamed('/welcome');
}