title: Unable to conditional hide tab by binding to data property in Ext.js 6
body: <p>I have a tabpanel where I'm capturing a shopping cart on tab one, payment information on tab two, and showing a summary on tab three. </p>
  
  <p>I would like to hide tab two if the shopping cart total is 0. </p>
  
  <p>The problem I'm running into is that when I try to bind to a formula that determines a boolean value for hiding the tab, the tab does not hide. </p>
  
  <p><a href="https://fiddle.sencha.com/#fiddle/1f7b" rel="nofollow">Here's a poc</a>:</p>
  
  <pre><code>Ext.define('TestPanel',{
      extend: 'Ext.tab.Panel',
      xtype: 'testpanel',
      title: 'Test Panel',
      viewModel:{
        data:{
            cartTotal: 0
        },
        formulas:{
            hideTabTwo: function(get){
                return get('cartTotal') == 0 ? true : false
            }
        }
      },
  
      items:[
          {
              title: 'Tab 1',
              items:[
                  {
                    xtype: 'textfield',
                    bind:{
                        value: '{cartTotal}'
                    }
                  },
                  {
                      bind:{html:'Cart Total: {cartTotal}'}
                  },
                  {
                      bind:{
                          html: 'Hide Tab 2: {hideTabTwo}'                
                      }
                  }
                ]
          },
          {
              title: 'Tab 2',
              html: 'omg',
              bind:{
                  hidden: '{hideTabTwo}'
              }
          },
          {
              title: 'Tab 3',
              html: 'lol'
          }
      ]
  })
  
  Ext.application({
      name : 'Fiddle',
  
  
  
  
      launch : function() {
          Ext.create('TestPanel',{
              renderTo: Ext.getBody()
          })
      }
  });
  </code></pre>
  
  <p>I can't see what's going wrong here. If you look at the output of the line <code>html: 'Hide Tab 2: {hideTabTwo}'</code> in the fiddle, you'll see that it is toggling between true and false. </p>
  
  <p>Any ideas?</p>