You should not add dojo.require("dijit.form.Button")
in onClientLoad
event. It serves a different purpose. Remove that code and add dijit.form.Button
as Dojo Module in Resources tab of your XPages properties. In source it should come like this:
<xp:this.resources>
<xp:dojoModule name="dijit.form.Button"></xp:dojoModule>
</xp:this.resources>