title: Populate combobox store depending on value in other combobox
body: <p>I'm having two comboboxes, one each for website and domain.
  What I need to do is on selecting a value in first dropdown, values in other dropdown should be filtered.
  I've written the following code :</p>
  
  <pre><code>var webSiteComboStore = Ext.create('Ext.data.Store', {
              fields : ['WebsiteNo','WebsiteName'],
              proxy : {
                  url : 'getListOfWebsites',
                  type : 'ajax'
              }
      });
  
  
  var win= Ext.create('Ext.window.Window',{
   layout: 'anchor',
   id:'myWin',
   width:500,
   modal:true,
   resizable:true,
   autoScroll:true,
   bodyPadding:'30px',
   title:"Add "+me.clicked.text,
  
   items:[{
    xtype:'combo',
    fieldLabel:'Website',
    emptyText:'Select',
    anchor:'90%',
    margin:'10 30 10 20',
    multiSelect : false,
    store : webSiteComboStore,
    editable:false,
    forceSelection:true,
    displayField : 'WebsiteName',
    valueField : 'WebsiteNo',
    submitEmptyText :'false',
    listeners:{
        'change':function(){
            var comboVal = this.value;
            this.up().items.items[1].bindStore(null);
            if(this.isDirty()){
              var  filteredDomainStore =   Ext.create('Ext.data.JsonStore',{
                  autoDestroy: true,
                  fields:['FilteredDomainName','FilteredDomainRefno'],        
  
                  proxy: {
                      type :'ajax',
                      url:'getListOfDomainsForWebsite.action',
                      extraParams:{
                          websiteRefno : comboVal
                      },
                      timeout:1000000,
                      reader:{
                          type:'json',
                          root:'domainsForWebsite'                
                  }       
                  }
                  });  
              this.up().items.items[1].bindStore(filteredDomainStore);
            }
  
          }
  
    }
  },{
        xtype:'combo',
        fieldLabel:'Domains',
        emptyText:'Select',
        anchor:'90%',
        margin:'10 30 10 20',
        multiSelect : false,
        store : null,
        editable:false,
        forceSelection:true,
        displayField : 'FilteredDomainName',
        valueField : 'FilteredDomainRefno',
        submitEmptyText :'false'
    }
  ]
  });
  </code></pre>
  
  <p><code>'webSiteComboStore'</code> is the store I've defined for the website combobox.
  On <code>'change'</code> event of first combobox, I'm creating the store for second (domain) combobox.</p>
  
  <p>For the first time, if I choose any value in first combobox, other combobox values are getting filtered.
  But when I go and choose some other value in first combo, this error shows up in console even before going and clicking on second combo</p>
  
  <pre><code>Uncaught TypeError: Cannot read property 'isSynchronous' of null
  </code></pre>
  
  <p>Needless to say, second combo shows previous values only and following error comes when clicked</p>
  
  <p><code>'Uncaught TypeError: Cannot read property 'findExact' of null'</code></p>
  
  <p>You can find the mentioned errors in the screenshot below</p>
  
  <p><a href="https://i.stack.imgur.com/P25GF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/P25GF.png" alt="enter image description here"></a></p>
  
  <p>I'm pretty new to ExtJS and the code I've written is mostly with the help of internet.
  Kindly give your suggestions.</p>