Ext.getCmp is not working

[ksaadain]

I am trying to create a combobox dynamically on page load, But while accessing the component in Javascript function "selecttop" by id, it is not returning the component. It always returns undefined. Please find below the code.

HTML :

<form id="form1" runat="server">
<ext:ResourceManager ID="ResourceManager1" runat ="server" />
<ext:Button ID="btnTest" runat="server" Text="Test" width="100" OnClientClick="selecttop();" ></ext:Button>

<script type="text/javascript">
var selecttop = function () {

var cnt = Ext.getCmp('cmbItem');
var store = cnt.getStore();

cnt.setValue(store.getAt(0).get('ItemID'));
};
</script>
</form>

Code :

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

Dim cmbItem As New Ext.Net.ComboBox

cmbItem.ID = "cmbItem"
cmbItem.Width = "200"
cmbItem.TypeAhead = True
cmbItem.ForceSelection = True
cmbItem.QueryMode = Ext.Net.DataLoadMode.Local
cmbItem.TriggerAction = Ext.Net.TriggerAction.All
cmbItem.DisplayField = "ItemTitle"
cmbItem.ValueField = "ItemID"

Me.Controls.Add(cmbItem)

LoadItem(cmbItem)

End Sub

Sub LoadItem(ByVal cnt As Ext.Net.ComboBox)

Dim store As New Ext.Net.Store
Dim model As New Ext.Net.Model

model.Fields.Add("ItemID")
model.Fields.Add("ItemTitle")

store.Model.Add(model)
store.DataSource = New Object() {
New Object() {"1", "Item 1"},
New Object() {"2", "Item 2"},
New Object() {"3", "Item 3"},
New Object() {"4", "Item 4"},
New Object() {"5", "Item 5"}
}

cnt.Store.Add(store)
store.Listeners.Load.Handler = "selecttop();"

End Sub