How can I reset a custom form control with Angular

When resetting the form, all controls receive null in the writeValue method. I can then reset the durationControl like that:

writeValue(obj: any): void {
    if (obj === null) {
        this.durationControl.reset();
    }

    this.durationControl.setValue(obj, {
        emitModelToViewChange: true
    });
    this.onChangeCallback(obj);
}