extjs3.4 : how to use a fieldset in 2 different forms?

I am using ExtJS 3.4, I have a fieldset mainDetailsFieldSet which I want to use in two forms, addFormPanel and updateFormPanel. I am able to get the fieldset in addFormPanel form, but I am not able to get it in updateFormPanel..I am getting a single blue line. I am not able to find what is wrong here...can someone help me with this?

Here is my code:

//mainfieldset with a textfield and combobox

  var clCombo = new Ext.form.ComboBox({
                    store: store,
                    fieldLabel: 'Name',
                    displayField: 'clName',
                    name: 'clName',
                    valueField: 'clName',
                    typeAhead: true,
                    mode: 'local',
                    triggerAction: 'all',
                    emptyText: 'Select Here'
          });
this.mainDetailsFieldSet = new Ext.form.FieldSet({
        title: 'Details',
        items:[
            {
                fieldLabel: ' Name',
                xtype: 'textfield',
                name: 'name'
                },clCombo
            ]
        });
var mainDetailsFieldSet = this.mainDetailsFieldSet ;

//addFormPanel, where I am using mainfieldset

this.addFormPanel = new Ext.form.FormPanel({
                title: 'Add Form',
                autoScroll: true,
                items:[
                    mainDetailsFieldSet ]
});

//updateformpanel, where I want to add the same field set again

this.updateFormPanel = new Ext.form.FormPanel({
            autoScroll: true,
            items:[mainDetailsFieldSet]
        }); 

Thanks in advance

You cannot render one instance at to different places.

Variant A: You will need to create a second instance if you need it twice.

this.comboCfg = {
      store: store,
      fieldLabel: 'Name',
      displayField: 'clName',
      name: 'clName',
      valueField: 'clName',
      typeAhead: true,
      mode: 'local',
      triggerAction: 'all',
      emptyText: 'Select Here'
};
this.mainDetailsFieldSet1 = new Ext.form.FieldSet({
    title: 'Details',
    items:[{
        fieldLabel: ' Name',
        xtype: 'textfield',
        name: 'name'
        },Ext.apply({xtype:'combo'},comboCfg)]
});
this.mainDetailsFieldSet2 = new Ext.form.FieldSet({
    title: 'Details',
    items:[{
        fieldLabel: ' Name',
        xtype: 'textfield',
        name: 'name'
        },Ext.apply({xtype:'combo'},comboCfg)]
});
var mainDetailsFieldSet1 = this.mainDetailsFieldSet1;
var mainDetailsFieldSet2 = this.mainDetailsFieldSet2;

this.addFormPanel = new Ext.form.FormPanel({
     title: 'Add Form',
     autoScroll: true,
     items:[mainDetailsFieldSet1]
});

this.updateFormPanel = new Ext.form.FormPanel({
    autoScroll: true,
    items:[mainDetailsFieldSet2]
}); 

Variant B: But what you can do is remove and add the instance each time.

this.addFormPanel = new Ext.form.FormPanel({
     title: 'Add Form',
     autoScroll: true
});
// before show
this.addFormPanel.add(mainDetailsFieldSet);
// before hide
this.addFormPanel.remove(mainDetailsFieldSet);

this.updateFormPanel = new Ext.form.FormPanel({
     autoScroll: true
});
// before show
this.updateFormPanel .add(mainDetailsFieldSet);
// before hide
this.updateFormPanel .remove(mainDetailsFieldSet);

---

Note Use configurations with xtype's as often as you can and don't define any id by yourself if it is not strictly needed.

---

Variant C:

this.comboCfg = {
      store: store,
      fieldLabel: 'Name',
      displayField: 'clName',
      name: 'clName',
      valueField: 'clName',
      typeAhead: true,
      mode: 'local',
      triggerAction: 'all',
      emptyText: 'Select Here'
};
this.mainDetailsFieldSetCfg = {
    xtype: 'fieldset',
    title: 'Details',
    items:[
       { xtype:'textfield',fieldLabel:' Name',name:'name'},
       Ext.apply({xtype:'combo'},comboCfg)
    ]
});

this.addFormPanel = new Ext.form.FormPanel({
     title: 'Add Form',
     autoScroll: true,
     items:[this.mainDetailsFieldSetCfg]
});

this.updateFormPanel = new Ext.form.FormPanel({
    autoScroll: true,
    items:[this.mainDetailsFieldSetCfg]
});