title: ReferenceError: instance is not defined
body: <p>This is my XML code. When I click a button it shows following error: </p>
  
  <blockquote>
    <p>ReferenceError: instance is not defined</p>
  </blockquote>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;templates id="template" xml:space="preserve"&gt;
    &lt;t t-extend="ListView.buttons"&gt;
      &lt;t t-jquery="button.o_list_button_add" t-operation="after"&gt;
        &lt;t t-if="widget.model=='ref.reference'"&gt;
          &lt;button class="btn btn-sm btn-default" type="button"&gt;Enable&lt;/button&gt;
          &lt;button class="btn btn-sm btn-primary disable_ref" type="button"&gt;Disable&lt;/button&gt;
        &lt;/t&gt;
      &lt;/t&gt;
    &lt;/t&gt;
  &lt;/templates&gt;
  </code></pre>
  
  <p>This is in javascript what wrong in this code:</p>
  
  <pre><code>var ListView = require('web.ListView');
  
  ListView.include({
      render_buttons: function() {
  
          // GET BUTTON REFERENCE
          this._super.apply(this, arguments)
          if (this.$buttons) {
             var btn = this.$buttons.find('.disable_ref')
          }
  
          // PERFORM THE ACTION
          btn.on('click', this.proxy('do_new_button'))
  
      },
      do_new_button: function() {
  
          instance.web.Model('ref.reference')
              .call('ref_dis', [[]])
      }
  })
  </code></pre>