Flutter - Error : IconData cant be assigned to the parameter type

Error: The argument type 'IconData' can't be assigned to the parameter type 'Widget?'.
 - 'IconData' is from 'package:flutter/src/widgets/icon_data.dart' ('../../../snap/flutter/common/flutter/packages/flutter/lib/src/widgets/icon_data.dart').
 - 'Widget' is from 'package:flutter/src/widgets/framework.dart' ('../../../snap/flutter/common/flutter/packages/flutter/lib/src/widgets/framework.dart').
                   icon: Icons.ac_unit,