Please add CausesValidation="false" for Button_Preview and check.
<asp:button id="Button_Preview" runat="server" causesvalidation="false" text="Preview" onclick="Button_Preview_Click" xmlns:asp="#unknown" />
or else use this
You are using validators,try this
<asp:requiredfieldvalidator id="valueReqValidator" runat="server" validationgroup="button" controltovalidate="btnValue" xmlns:asp="#unknown"></asp:requiredfieldvalidator>