Can not read parent node of ngFor generated element

I am trying to create a backdrop-click function, which should read the parent of my targeted element. This works so far, but when I want to read the parent of an, ngFor generated, element I get null.

onMouseDown(event: MouseEvent) {
if(event.target === this.elRef.nativeElement || this.isDescendant(this.elRef.nativeElement, event.target)) {
            if (this.state === 'OPEN') 
                this.state = 'CLOSE';
            else 
                this.state = 'OPEN';
            this.toggle.emit();
            return;
}

isDescendant(parent: any, child: any) {
        let node: any = child.parentNode; // does not work on ngFor generated elements

        while (node !== null) {
            if (node === parent) {
                return true;
            }
            node = node.parentNode;
        }

        return false;
}

Maybe someone knows how to avoid this problem? Thank you in advance!

best regards Jonaz