DefaultValueAccessor not possible for ionic?

Ionic Framework ionic-v3
1

I try to create a custom DefaultValueAccessor. The Accessor should make a empty string to undefined.
How is it possible ?

Here is my code, that is not working.

import { Directive, forwardRef, Renderer2, ElementRef } from '@angular/core';
import { DefaultValueAccessor, NG_VALUE_ACCESSOR } from '@angular/forms';

export const VALUE_ACCESSOR: any = {
	provide: NG_VALUE_ACCESSOR,
	useExisting: forwardRef(() => InputExtensionValueAccessor),
	multi: true
};

@Directive({
	selector: `
	ion-input:not([type=checkbox]):not([type=radio]):not([type=password]):not([readonly]),
	ion-textarea:not([readonly])[formControlName],
	ion-textarea:not([readonly])[formControl],
	ion-textarea:not([readonly])[ngModel]
  `,
	host: {
		'(input)': '$any(this)._handleInput($event.target.value)',
		'(blur)': 'onTouched()',
		'(compositionstart)': '$any(this)._compositionStart()',
		'(compositionend)': '$any(this)._compositionEnd($event.target.value)'
	},
	providers: [VALUE_ACCESSOR]
})
export class InputExtensionValueAccessor extends DefaultValueAccessor {
	// HACK: Allows use of DefaultValueAccessor as base
	// (https://github.com/angular/angular/issues/9146)
	static decorators = undefined;

	constructor(renderer: Renderer2, elementRef: ElementRef) {
		// tslint:disable-next-line:no-null-keyword
		super(renderer, elementRef, (null as any));
	}

	registerOnChange(fn: (_: any) => void): void {
		super.registerOnChange(value => {
			fn(value === '' ? undefined : value);
		});
	}
}