The parameter 'key' can't have a value of 'null' and The parameter 'title' can't have a value of 'null'

I have got two errors

1.  Error: The parameter 'key' can't have a value of 'null' because of its type 'Key', but the implicit default value is 'null'.

and 

2. Error: The parameter 'title' can't have a value of 'null' because of its type 'String', but the implicit default value is 'null'.

This was my part of code

class MyHomePage extends StatelessWidget {

    MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

I have changed this code to 

class MyHomePage extends StatelessWidget {

  MyHomePage({Key? key, this.title=''}) : super(key: key);

  final String title;

and now it works.