Angular 4 error: Cannot read property 'push' of undefined

I am working on an Angular 4 app, where I am trying to push data into an array, but keep getting the error ERROR TypeError: Cannot read property 'push' of undefined even though I initialized the array.

Here's the code:

import {Component, OnInit} from '@angular/core';

declare let d3:any;

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
export class AppComponent implements OnInit {

  //Initialized array here
  networkLinkList: Array<any> = [];
  ngOnInit() {
    this.networkGraph();
  }

  networkGraph() {
    d3.netJsonGraph("../assets/json/network.json",
      {
        onClickNode: (url, opts) => {
          this.highlightNodes(url, "nodes", true);
        }
      }
    );
  }

  highlightNodes(url, type, initial) {
    let url_children = url.children;

    if(type === "nodes") {
      let clicked_node_object = {
                                  name: url.name,
                                  description: url.description,
                                  level: url.level,
                                  link: url.url
                                };
      //Push here is working fine
      this.networkLinkList.push(clicked_node_object);
      //Can see the array values here
      console.log(this.networkLinkList);

      d3.selectAll(".njg-node").filter(function(d) {

        if(url_children.length > 0) {
          for(let child=0; child<url_children.length; child++) {
            if(d.id == url_children[child]) {

              //Can see the all the values here
              console.log(d.name + ", " + d.description + ", " + d.level + ", " + d.url);
              let child_node_object = {
                name: d.name,
                description: d.description,
                level: d.level,
                link: d.url
              };

              //Push here is throwing the error
              this.networkLinkList.push(child_node_object);
            }
          }
        }
        return ((url.children.length > 0 && (d.level === url.level + 1)));
      });
     }
}

Please help me resolve this issue, as I am not sure why the error is being thrown, even if the array is initialized.