title: Child nodes does not display for angular-tree-component
body: <p>I tried implementing angular-tree-component from basic example provided => <a href="https://angular2-tree.readme.io/docs#basic-usage" rel="nofollow noreferrer">Angular Tree Example</a></p>
  
  <p>The issue is after implementation i am only able to see only root nodes and that too without expand(child roots are not visible). There is no error as such in console.</p>
  
  <p>Component:</p>
  
  <pre><code> @Component({
    selector: 'app-category',
    templateUrl: './category.component.html',
    styleUrls: [
      './category.component.css'
    ]
  })
  export class CategoryComponent implements OnInit {
  
    nodes: any;
    options = {};
    showpanel: boolean;
    categories: CategoriesModel[];
    constructor(private adminService: AdminService) {
  
    }
    ngOnInit() {
      this.showpanel = false;
      this.categories = [];
      this.adminService.getCategoryData()
        .subscribe(
          (catData: CategoriesModel[]) =&gt; {
            this.categories = catData;
            this.showpanel = true;
          });
          this.nodes =  [
            {
              id: 1,
              name: 'root1',
              children: [
                { id: 2, name: 'child1' },
                { id: 3, name: 'child2' }
              ]
            },
            {
              id: 4,
              name: 'root2',
              children: [
                { id: 5, name: 'child2.1' },
                {
                  id: 6,
                  name: 'child2.2',
                  children: [
                    { id: 7, name: 'subsub' }
                  ]
                }
              ]
            }
          ];
    }
  }
  </code></pre>
  
  <p>HTML:</p>
  
  <pre><code>    &lt;div &gt;
    &lt;tree-root [nodes]="nodes" [options]="options"&gt;&lt;/tree-root&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>CSS:</p>
  
  <pre><code>    .tree-children.tree-children-no-padding { padding-left: 0 }
  .tree-children { padding-left: 20px; overflow: hidden }
  .node-drop-slot { display: block; height: 2px }
  .node-drop-slot.is-dragging-over { background: #ddffee; height: 20px; border: 2px dotted #888; }
  .toggle-children-wrapper-expanded .toggle-children { transform: rotate(90deg) }
  .toggle-children-wrapper-collapsed .toggle-children { transform: rotate(0); }
  .toggle-children-wrapper {
    padding: 2px 3px 5px 1px;
  }
  .toggle-children-placeholder {
    display: inline-block;
    height: 10px;
    width: 10px;
    position: relative;
    top: 1px;
    padding-right: 3px;
  }
  .node-content-wrapper {
    display: inline-block;
    padding: 2px 5px;
    border-radius: 2px;
    transition: background-color .15s,box-shadow .15s;
  }
  .node-wrapper {display: flex; align-items: flex-start;}
  .node-content-wrapper-active,
  .node-content-wrapper.node-content-wrapper-active:hover,
  .node-content-wrapper-active.node-content-wrapper-focused {
    background: #beebff;
  }
  .node-content-wrapper-focused { background: #e7f4f9 }
  .node-content-wrapper:hover { background: #f7fbff }
  .node-content-wrapper-active, .node-content-wrapper-focused, .node-content-wrapper:hover {
    box-shadow: inset 0 0 1px #999;
  }
  .node-content-wrapper.is-dragging-over { background: #ddffee; box-shadow: inset 0 0 1px #999; }
  .node-content-wrapper.is-dragging-over-disabled { opacity: 0.5 }
  
  tree-viewport {
    -webkit-tap-highlight-color: transparent;
    height: 100%;
    overflow: auto;
    display: block;
  }
  .tree-children { padding-left: 20px }
  .empty-tree-drop-slot .node-drop-slot { height: 20px; min-width: 100px }
  .angular-tree-component {
    width: 100%;
    position:relative;
    display: inline-block;
    cursor: pointer;
    -webkit-touch-callout: none; /* iOS Safari */
    -webkit-user-select: none;   /* Chrome/Safari/Opera */
    -khtml-user-select: none;    /* Konqueror */
    -moz-user-select: none;      /* Firefox */
    -ms-user-select: none;       /* IE/Edge */
    user-select: none;           /* non-prefixed version, currently not supported by any browser */
  }
  
  tree-root .angular-tree-component-rtl {
    direction: rtl;
  }
  tree-root .angular-tree-component-rtl .toggle-children-wrapper-collapsed .toggle-children {
    transform: rotate(180deg) !important;
  }
  tree-root .angular-tree-component-rtl .tree-children {
    padding-right: 20px;
    padding-left: 0;
  }
  
  tree-node-checkbox {
    padding: 1px;
  }
  </code></pre>
  
  <p>Display in Browser:</p>
  
  <p><a href="https://i.stack.imgur.com/nsH4d.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nsH4d.png" alt="Display in Browser"></a></p>