Error:object reference is not set to instance of an object at string b. i need to pass radiobutton selected value in my repeater from mock.aspx to mock1.aspx

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage1.master" AutoEventWireup="true" CodeFile="mock.aspx.cs" Inherits="mock" %>


<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    


<div>
<asp:Repeater ID="RepDetails" runat="server">
<HeaderTemplate>
<table style=" border:1px solid #df5015; width:500px" cellpadding="0">
<tr style="background-color:#df5015; color:White">
<td colspan="2">
<b>Questions:</b>
</td>
</tr>
</HeaderTemplate>
<itemtemplate>
<tr style="background-color:#EBEFF0">
<td>
<table style="background-color:#EBEFF0;border-removed1px dotted #df5015; width:500px">
<tr>
<td>

    <asp:Label ID="Label1" runat="server" Text='<%#Eval("qno") %>' Font-Bold="true"/>

<asp:Label ID="lblSubject" runat="server" Text='<%#Eval("question") %>' Font-Bold="true"/>
</td>
</tr>
</table>
</td>
</tr>
<tr>

</tr>
<tr>
<td>
<table style="background-color:#EBEFF0;border-removed1px dotted #df5015;border-removed1px solid #df5015; width:500px">
<tr>
<td>
    <asp:RadioButton ID="RadioButton1" groupname="radio" Text='<%#Eval("option1") %>' EnableViewState="false" runat="server"/><br />
        <asp:RadioButton ID="RadioButton2" groupname="radio" Text='<%#Eval("option2") %>' EnableViewState="false" runat="server"/><br />
    <asp:RadioButton ID="RadioButton3" groupname="radio"  Text='<%#Eval("option3") %>' EnableViewState="false" runat="server"/><br />
    <asp:RadioButton ID="RadioButton4" groupname="radio" Text='<%#Eval("option4") %>' EnableViewState="false" runat="server"/>

          </td>
</tr>
</table>
</td>
</tr>
<tr>
<td colspan="2">&nbsp;</td>
</tr>
</itemtemplate>
<footertemplate>
</footertemplate></table>


</div>
    <div>&lt;asp:Button id="btnnext" runat="server" Text="Next" OnClick="btnnext_Click"/&gt;&lt;asp:Label ID="lblvalue" runat="server"&gt;</div>

  protected void Page_Load(object sender, EventArgs e)
    {

        if (!IsPostBack)
        {
            BindRepeaterData();
        }
    }

protected void BindRepeaterData()
{
    SqlConnection con=new SqlConnection(connectionString);
    
    con.Open();
    SqlCommand cmd = new SqlCommand("select qno,question,option1,option2,option3,option4 from questionstable where qno BETWEEN 3 AND 6", con);
    DataSet ds = new DataSet();
    SqlDataAdapter da = new SqlDataAdapter(cmd);
    da.SelectCommand = cmd;
    da.Fill(ds);
    RepDetails.DataSource = ds;
    RepDetails.DataBind();
    con.Close();
}

protected void btnnext_Click(object sender, EventArgs e)
{
     foreach(RepeaterItem item in RepDetails.Items)             
            {
                RadioButton box = (RadioButton)item.FindControl("radio");
                string b = box.Text;    
         lblvalue.Text=b;  
 
             }   
    Response.Redirect("mock1.aspx?value="+lblvalue.Text);


        
    }

 void Page_Load(object sender, EventArgs e)
    {
        Label1.Text = Request.QueryString["value"].ToString();
}