nullreferenceexception object reference not set to an instance of an object in asp net

Line 2383:    protected void Page_UnLoad(object sender, EventArgs e)
Line 2384:    {Line 2385:        if (rptdoc.IsLoaded)Line 2386:        {
Line 2387:            rptdoc.Clone();

i have installed Sql server 2008 r2 and crystal report version13.

please help me to resolve this problem