Target of URI doesn't exist

I've created a Custom Widget and clicked on the green button on the top right View Boilerplate Code. the resulting window provides a Copy To Editor button, which I press. the system pastes this code to my editor:

import '/flutter_flow/flutter_flow_theme.dart';
import '/flutter_flow/flutter_flow_util.dart';
import '/custom_code/widgets/index.dart'; // Imports other custom widgets
import '/custom_code/actions/index.dart'; // Imports custom actions
import '/flutter_flow/custom_functions.dart'; // Imports custom functions
import 'package:flutter/material.dart';

and immediate all of these lines are underlined in red with the message posted as the Title of this question. so this stuff doesn't work out of the box. what am I supposed to do?