Render: "tree" not working

Hello,

I got the PRO version of the scheduler timeline.
I want to group lines with render: “tree”
Unfortunatly it’s not working, it only show 2 normal lines:
Production Department
Sales and Marketing

Here is my code :

 scheduler.skin = 'material';
scheduler.config.header = [
  'date',
  { html: "+", click: () => { this.zoomIn(); }, css: "dhx_zoom_in_button" },
  { html: "-", click: () => { this.zoomOut(); }, css: "dhx_zoom_out_button" },
  'prev',
  'today',
  'next'
];
scheduler.config.readonly = true;

scheduler.createTimelineView({
  name: "timeline",
  render: "tree",             // view mode
  x_unit: "day", // measuring unit of the X-Axis.
  x_date: "%d/%m",  // date format of the X-Axis
  x_step: 1,       // X-Axis step in 'x_unit's
  x_size: 180,       // X-Axis length specified as the total number of 'x_step's
  x_start: 0,      // X-Axis offset in 'x_unit's
  x_length: 7,     // number of 'x_step's that will be scrolled at a time
 // y_unit:          // sections of the view (titles of Y-Axis)
 //   scheduler.serverList("groups"),

  y_unit: [
    {
      key: "production", label: "Production Department", children: [
        {
          key: "p1", label: "Managers", children: [
            { key: "pm1", label: "John Williams" },
            { key: "pm2", label: "David Miller" }
          ]
        },
        { key: "p2", label: "Linda Brown" },
        { key: "p3", label: "George Lucas" }
      ]
    },
    {
      key: "sales", label: "Sales and Marketing", children: [
        { key: "s1", label: "Kate Moss" },
        { key: "s2", label: "Dian Fossey" }
      ]
    }
  ],

  y_property: "trainer_id", // mapped data property
  
  scrollable: true,
  column_width: 210,
  //the cell interval will be daytime from 8.00 till 18.00
  first_hour: 8,
  last_hour: 18,
  second_scale: {
    x_unit: "week", // the measuring unit of the axis (by default, 'minute')
    x_date: "Week %W" //the date format of the axis ("July 01")
  }
});

ok sorry, I forgot to add
import ‘dhtmlx-scheduler/codebase/ext/dhtmlxscheduler_treetimeline.js’;
sorry