It looks like injector.get(NgControl) is throwing a deprecation warning, so I wanted to chime in with another solution:
constructor(public ngControl: NgControl) {
ngControl.valueAccessor = this;
}
The trick is to also remove NG_VALUE_ACCESSOR from the providers array otherwise you get a circular dependency.
More information about this is in this talk by Kara Erickson of the Angular team.