Confusion Getting this Errorr Unable to get property 'checked' of undefined or null reference

In Function when i use .checked then it stop working & generate below Error. Currently im using Visual Studio 2015.Please Help

function GenderValidation(sender , e)
    {
        var male = document.getElementById('RadioButton_male').Checked;
        var female = document.getElementById('RadioButton_female').Checked;
        if (male == true || female == true)
        {
            e.IsValid = true;
        } else {
            e.IsValid = false;
        }

    }

**

• 0x800a138f - JavaScript runtime error: Unable to get property 'checked' of undefined or null reference

**

This is my Complete code

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="contactus.aspx.cs" Inherits="DVManagmentProject.contactus" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
    <style type="text/css">
        .auto-style1 {
            width: 70%;
        }
        .auto-style2 {
            width: 190px;
        }
        .auto-style3 {
            width: 190px;
            height: 34px;
        }
        .auto-style4 {
            height: 34px;
        }
        .auto-style5 {
            width: 190px;
            height: 150px;
        }
        .auto-style6 {
            height: 150px;
        }
    </style>
    <script type="text/javascript"  >
        function GenderValidation(sender , e)
        {
            var male = document.getElementById('ContentPlaceHolder1_RadioButton_male').Checked;
            var female = document.getElementById('ContentPlaceHolder1_RadioButton_female').Checked;
            if (male == true || female == true)
            {
                e.IsValid = true;
            } else {
                e.IsValid = false;
            }

        }

        function CustomValidatorAgremennt(sender, e)
        {

            var agreemenT = document.getElementById('ContentPlaceHolder1_CheckBox_agreement').Checked;
            if(agreemenT==true)
            {
                e.IsValid = true;
            }else
            {
                e.IsValid = false;
            }

        }
</script>
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
    <h2>
        This is Contact Us Page</h2>
                <asp:ValidationSummary ID="contact_ValidationSummary" runat="server" BackColor="#FFFF99" ForeColor="Black" Font-Bold="true" Width="694px" BorderStyle="Double" />
    <h3> Please Fill the Form Below:</h3>
    <br />
    <table class="auto-style1">
        <tr>
            <td class="auto-style2">Name:</td>
            <td>
                <asp:TextBox ID="name_TextBox" runat="server" Width="320px"></asp:TextBox>
                <asp:RequiredFieldValidator ID="RequiredFieldValidator_name" 
                    runat="server"  Text="*"
                    ErrorMessage="Please Enter Your Name" 
                    ForeColor="Red" 
                    ControlToValidate="name_TextBox" 
                    Display="Dynamic"
                     > </asp:RequiredFieldValidator>

                <%-- <asp:RangeValidator ID="RangeValidator_name" runat="server" 
                    ErrorMessage="*Text should between 10 to 30" MinimumValue="10" MaximumValue="30"
                    Type="Integer" ControlToValidate="name_TextBox"
                    Display="Dynamic"></asp:RangeValidator>--%>
            </td>
        </tr>
        <tr>
            <td class="auto-style3">Email:</td>
            <td class="auto-style4">
                <asp:TextBox ID="email_TextBox" runat="server" Width="320px"></asp:TextBox>
                <asp:RequiredFieldValidator ID="RequiredFieldValidator_email" 
                    runat="server"  
                    SetFocusOnError="true"
                     Text="*" 
                    ErrorMessage="Please Enter Your Email Name" 
                    ControlToValidate="email_TextBox" 
                    ForeColor="Red" 
                    Display="Dynamic"> </asp:RequiredFieldValidator>
                <asp:RegularExpressionValidator 
                    ID="RegularExpressionValidator_email_TextBox" 
                    runat="server" 
                    ErrorMessage="Please Enter Correct Email Format"
                    Display="Dynamic"
                    ControlToValidate="email_TextBox"
                    ForeColor="Red"
                    SetFocusOnError="true"
                     Text="*"
                     ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" > </asp:RegularExpressionValidator>

            </td>
        </tr>
        <tr>
            <td class="auto-style3">Re-Enter Email:</td>
            <td class="auto-style4">

                <asp:TextBox ID="TextBox_ReEmail" runat="server" Width="320px"></asp:TextBox>

                 <asp:CompareValidator 
                    ID="CompareValidator_ReEmail_TextBox" 
                    runat="server"
                    ErrorMessage="Email Must be Match"
                    Operator="Equal"
                    ControlToValidate="TextBox_ReEmail"
                    ControlToCompare="email_TextBox"
                    Type="String"
                     Display="Dynamic"
                     ForeColor="Red"
                     Font-Bold="true">*</asp:CompareValidator>

                <asp:RequiredFieldValidator ID="RequiredFieldValidator_ReEmail" 
                    runat="server"  
                    SetFocusOnError="true"
                     Text="*" 
                    ErrorMessage="Please Confirm Your Email Name" 
                    ControlToValidate="TextBox_ReEmail" 
                    ForeColor="Red" 
                    Display="Dynamic"> </asp:RequiredFieldValidator>
                <asp:RegularExpressionValidator 
                    ID="RegularExpressionValidator_ReEmail" 
                    runat="server" 
                    ErrorMessage="Please Enter Correct Re-Email Format"
                    Display="Dynamic"
                    ControlToValidate="TextBox_ReEmail"
                    ForeColor="Red"
                    SetFocusOnError="true"
                     Text="*"
                     ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" ></asp:RegularExpressionValidator>
            </td>
        </tr>
        <tr>
            <td class="auto-style3">Gender:</td>
            <td class="auto-style4">

                <asp:RadioButton ID="RadioButton_male" runat="server" Text="Male" GroupName="GenderGroup" />
                <asp:RadioButton ID="RadioButton_female" runat="server" Text="Female" GroupName="GenderGroup" />
                <asp:CustomValidator 
                    ID="CustomValidator_genders" 
                    runat="server" 
                    ErrorMessage="Please Select Gender.!" 
                    Font-Bold="True" 
                    Display="Dynamic" ForeColor="Red" OnServerValidate="CustomValidator_genders_ServerValidate">*</asp:CustomValidator>
            </td>
        </tr>
        <tr>
            <td class="auto-style3">Age:</td>
            <td class="auto-style4">

                <asp:TextBox ID="Age_TextBox" runat="server" Width="318px"></asp:TextBox>

                   <asp:RequiredFieldValidator ID="RequiredFieldValidator1" 
                    runat="server"  
                    SetFocusOnError="true"
                     Text="*" 
                    ErrorMessage="Please Enter Your Age" 
                    ControlToValidate="Age_TextBox" 
                    ForeColor="Red" 
                    Display="Dynamic"> </asp:RequiredFieldValidator>

                <asp:RangeValidator 
                    ID="RangeValidator_Age"
                    runat="server" 
                    ErrorMessage="Age should between 18 to 99"
                    Type="Integer"
                    Display="Dynamic"
                    SetFocusOnError="true"
                    ControlToValidate="Age_TextBox"
                    MinimumValue="18"
                    MaximumValue="99" 
                    ></asp:RangeValidator>

            </td>
        </tr>
        <tr>
            <td class="auto-style2">Subject:</td>
            <td>
                <asp:DropDownList ID="subject_DropDownList" runat="server" Width="320px">
                </asp:DropDownList>
            </td>
        </tr>
        <tr>
            <td class="auto-style5">Message </td>
            <td class="auto-style6">
                <asp:TextBox ID="message_TextBox" runat="server" Height="140px" TextMode="MultiLine" Width="320px"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td class="auto-style2">Agreement:</td>
            <td>
                <asp:CheckBox ID="CheckBox_agreement" runat="server" Text="Please Select Our Terms And Conditions.!" />
                    <asp:CustomValidator 
                        ID="CustomValidator_Agreement" 
                        runat="server" 
                        Display="Dynamic"
                        ErrorMessage="Please Check Terms and Conditions" 
                        Font-Bold="True" 
                        ForeColor="Red" OnServerValidate="CustomValidator_Agreement_ServerValidate">*</asp:CustomValidator>
            </td>
        </tr>
        <tr>
            <td class="auto-style2">&nbsp;</td>
            <td>
                <asp:Button ID="submit_Button" runat="server" Font-Bold="True" Text="Submit" Width="320px" />
            </td>
        </tr>
        <tr>
            <td colspan="2">
                &nbsp;</td>
        </tr>
    </table>
    <p>
        &nbsp;</p>
</asp:Content>

This is Debbuger Code

  [1]: https://i.stack.imgur.com/PG8hC.jpg
  [2]: https://i.stack.imgur.com/Jqw0X.jpg
  [3]: https://i.stack.imgur.com/DuaNT.jpg
  [4]: https://i.stack.imgur.com/DrTPP.jpg
  [5]: https://i.stack.imgur.com/1Dhc1.jpg
  [6]: https://i.stack.imgur.com/880dI.jpg
  [7]: https://i.stack.imgur.com/DWQ7D.jpg
  [8]: https://i.stack.imgur.com/LdHXg.jpg
  [9]: https://i.stack.imgur.com/xVXI1.jpg
  [10]: https://i.stack.imgur.com/qC4LA.jpg

Problem is Solved . I have Changed

ContentPlaceHolder1_RadioButton_male and ContentPlaceHolder1_RadioButton_female

instead of

RadioButton_male and RadioButton_female

as well as in Javascript Function .i have changed (.Checked) in small (.checked) and it worked for me .