title: How to use this.field_manager.set_field_value
body: <p>I want to modify some values using "field_manager" of <code>FormWidget</code>.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>openerp.new_payrolls = function (instance) {
     instance.web.client_actions.add('example.action','instance.new_payrolls.Action');
     instance.new_payrolls.Action = instance.web.Widget.extend({
         template: 'new_payrolls.action',
         events: { 
            'click .oe_web_example_file button': 'Upload',
            'click .oe_web_example_load button': 'momo_change', 
         }, 
         init: function () {
             this._super.apply(this, arguments);
          var self = this;
          this._start = null;
          this._watch = null;
          var Model = new instance.web.Model('new_payrolls'); 
        },
        start: function () {
            this._super();
            console.log('Data Loading');
            var self = this;
            this.field_manager.on("field_changed:matricule", this);
            var data = this.field_manager.get_field_value("matricule"); // matricule is an attribute from new_payrolls class
       }, 
  }
  </code></pre>
  
  <p>Then i want to use `this.field_manager.set_values(matricule).
  Error: : Uncaught TypeError: Cannot read property 'on' of undefined.</p>