A null value for parameter 'id' of non-nullable type 'System.Int32' for method 'Test1.Models.Event FormView1_GetItem(Int32)' in 'Test1.EventDetails'. An optional parameter must be a reference type or a nullable type.

I am new at working with web api and getting data from there.  this is my error
A null value for parameter 'id' of non-nullable type 'System.Int32' for method 'Test1.Models.Event FormView1_GetItem(Int32)' in 'Test1.EventDetails'. An optional parameter must be a reference type or a nullable type.
 
I've got one web application and one web api .

this is the code for the formview , can somebody tell me how to fix this error !

.aspx

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="EventDetails.aspx.cs" Inherits="Test1.EventDetails" %>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
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