title: multilevel nested list in material-ui next
body: <p>I'd like to create multilevel nested list to display the menu on the left side - - very similar as on oficial site: <a href="https://material-ui-next.com/" rel="noreferrer">https://material-ui-next.com/</a>.
  data source is in JSON where for each item there's also information about parent item and some other data - - this is the example:</p>
  
  <pre><code>{
      "task_number": 1201092,
      "task": "Monthly Cash Flow from Deliveries",
      "task_parent_number": 1201090,
      "task_parent": "Wholesale Cash Flow"
  },
  {
      "task_number": 1201095,
      "task": "Monthly Cash Flow from Fix Amounts",
      "task_parent_number": 1201090,
      "task_parent": "Wholesale Cash Flow"
  },
  {
      "task_number": 1201100,
      "task": "Wholesale Positions",
      "task_parent_number": 1200007,
      "task_parent": "Wholesale Contract Portfolio"
  },
  {
      "task_number": 1201200,
      "task": "All Wholesale Positions",
      "task_parent_number": 1201100,
      "task_parent": "Wholesale Positions"
  }
  </code></pre>
  
  <p>I'am able to create an object with various nested elements - children - if they exist with followin function:</p>
  
  <pre><code>function getNestedChildren(arr, parent) {
    var out = [];
    for (var i in arr) {
      if (arr[i].task_parent_number == parent) {
        //console.log(i, arr[i].task_parent_number, arr[i].task_number);
        var children = getNestedChildren(arr, arr[i].task_number);
        if (children.length) {
          arr[i].children = children;
        }
        out.push(arr[i]);
      }
    }
  
    return out;
  }
  </code></pre>
  
  <p>I've been following instructions for creating a nested list and importing it here: <a href="https://material-ui-next.com/demos/lists/#nested-list" rel="noreferrer">https://material-ui-next.com/demos/lists/#nested-list</a></p>
  
  <p>..but I am not able to create a menu with nested elements as desired . . if someone can point me in the right direction would be great..</p>