Adding a message box to a button icon

Adding a message box to a button icon

I am trying to add a message box when the user clicks on a certain button.
This is the code that I have:
{
text: 'Button Icon',
id: 'buttonIcon',
cls: 'x-btn-text-icon',
launch: function() {
Ext.MessageBox.show({
title: "",
msg: "Message Text",
icon: Ext.MessageBox.WARNING,
buttons: Ext.MessageBox.OKCANCEL,
fn: function(buttonIcon) {
if (buttonIcon === "ok") {
alert("Done!")
}
}
});
}
Right mow, when you click on the button icon nothing happens at all and I
need it to display the message that I have entered. Please Help.