I have a Combobox. When I select a list item the first time it works fine, when I select another item it throws this error in firebug (this.hiddenField is undefined) :

            items: [
                    {
                        columnWidth: 0.5,
                        layout: 'form',
                        bodyStyle: 'background: none;margin:0 1px;',
                        title: 'Data View',
                        frame: true,

                        items: [
                            new Ext.form.ComboBox({
                                id: 'QuestionSelector',
                                store: qStore,
                                hidden: false,
                                mode: 'local',                                
                                forceSelection: true,
                                triggerAction: 'all',
                                selectOnFocus: true,
                                fieldLabel: 'Select question',
                                emptyText: 'Select question...',
                                editable: false,
                                listWidth: 300,
                                listeners: {
                                    select: function(c, d, i) {
                                        if (d.data.value) {
                                            rpc = new JsonRpc({
                                                url: GlobalConstants.chartControlsUrl
                                            }); // eo call

                                            rpc.call({
                                                method: 'SetQuestions',
                                                parameters: [NavTree.selectedNode.id, [d.data.value]],
                                                callback: function(success, data, response) {
                                                    if (success && data instanceof Object) {
                                                        var aTab = MainBodyTabs.getActiveTab();
                                                        if (aTab)
                                                            aTab.Content.refresh(aTab);
                                                    }
                                                },
                                                scope: this
                                            });

                                        } // eo if

                                    }
                                }
                            })
                        ]
                    },