Don't know what file Error is referring to in flutter_local_notifications plugin

Add your icon to [projectFolder]/android/app/src/main/res/drawable (for example app_icon.png) and use that name here:

var initializationSettingsAndroid =
    new AndroidInitializationSettings('app_icon');

Hope you have solved it with above answered method, but I have solved with another method by using existing app icon.

var initializationSettingsAndroid =
    new AndroidInitializationSettings('@mipmap/ic_launcher');