Error says cannot read property of NULL for all elements

I am trying to create a request sting and add the value of a check box (var = PrintDetails) I do not know hwy my application does not see the check box. it just says cannot read property 'Checked' of NULL //JavaScript Code

$(document).ready(function () { console.log("Im ready now!"); });

function updateReport(response, responseStatus, request) { console.log('Updating Report...'); $("#InformReportGeneratedDiv").css("display", "block"); }

function generateReportCallout(requestString) { alert(requestString); $.ajax({ type: 'POST', url: url, data: requestString, success: updateReport, error: function (response) { alert("Something went wrong!"); } }); }

function generateEmptyLocationsUnassignedReport() { console.log("Generating Empty Locations Unassigned report..."); var printDetails = document.getElementById('BodyContent_Content_PrintDetails').checked ? "True" : "False"; var requestString = appname + "&PRGNAME=Empty Locations Unassign Cover" + "&ARGUMENTS=-L" + printDetails; generateReportCallout(requestString); }

//.aspx page <asp:Content ID="Content1" ContentPlaceHolderID="MainContent" runat="server"> <script type="text/javascript" src="JS/reports.js"></script> <div class="row"> <div class="col text-left text-center"> <h2><%: Title %></h2> <br /> </div> </div> <div id="InformReportGeneratedDiv" style="display: none;"> <h1>Your Report is being generated. Visit 'My Reports' Page to view it when it is ready.</h1> </div> <div class="text-center"> <asp:DropDownList ID="ReportDDL" runat="server" AutoPostBack="true" Font-Size="X-Large" Width="300px" Height="50px"> <asp:ListItem Selected="True" Value="">/asp:ListItem <asp:ListItem Value="Unassigned Locations aka Locations where Part Number field is empty">Empty Locations Unassigned/asp:ListItem /asp:DropDownList </div>

<br />
<h1 class="text-center"><%: ReportDDL.SelectedItem.Value %></h1>

<asp:Panel Visible='<%# String.Equals(ReportDDL.SelectedItem.Value, "Unassigned Locations aka Locations where Part Number field is empty") %>' runat="server" ID="GenerateEmptyLocUnassignedPanel">
    <div class="container-fluid">
        <div class="row pad-t-20">
            <div class="col-md-4 col-md-offset-4 text-center">
                <label class="two-em-text">Print Details?</label><asp:CheckBox ID="PrintDetails" runat="server" />
            </div>
        </div>
        <div class="row pad-t-20">
            <div class="col-md-2 col-md-offset-5 text-center">
                <input type="button" value="Generate Report" class="btn btn-primary btn-large" onclick="generateEmptyLocationsUnassignedReport();">
            </div>
        </div>
    </div>
</asp:Panel>

<div class="text-center pad-t-40">
    <a href="MyReports" runat="server" class="btn btn-default" role="button">View my reports</a>
</div>

/asp:Content