Error: 'this._targetEl.value.length' is null or not an object ASP.NET
I'm trying to bind the selected value from the date picker to an asp textbox but I have this Error: 'this._targetEl.value.length' is null or not an object. 

Here's the code: 

    <InsertItemTemplate>
        Book Title:
        <asp:DropDownList ID="DropDownList1" runat="server" 
            DataSourceID="booktitleDataSource" DataTextField="booktitle" 
            DataValueField="bookid" SelectedValue='<%# Bind("bookid", "{0}") %>'>
        </asp:DropDownList>
        <asp:SqlDataSource ID="booktitleDataSource" runat="server" 
            ConnectionString="<%$ ConnectionStrings:LibrarySystemConnectionString %>" 
            SelectCommand="SELECT [bookid], [booktitle] FROM [TblBooks]">
        </asp:SqlDataSource>
        <br />
        
        Employee PIN:
        <asp:TextBox ID="employeeidTextBox" runat="server" 
            Text='<%# Bind("employeeid") %>' />
        <br />
        
        Department:
        <asp:TextBox ID="departmentTextBox" runat="server" 
            Text='<%# Bind("department") %>' />
        <br />
        
        Date borrowed:
        <asp:TextBox ID="dateborrowedTextBox" runat="server" Text='<%# Bind("dateborrowed") %>' /> 
    <%--<input type="text" name="reservedateTextBox" readonly="readonly" id="reservedateTextBox">--%> 
        <a href="#" onclick="cdp1.showCalendar(this, 'dateborrowedTextBox'); return false;">Date Picker</a>       
        <br />
        
        <asp:Button ID="InsertButton" runat="server" CausesValidation="True" 
            CommandName="Insert" Text="Insert" />
        <asp:Button ID="InsertCancelButton" runat="server" CausesValidation="False" 
            CommandName="Cancel" Text="Cancel" />
    </InsertItemTemplate>

It was working when I 
tried using `<input type="text" name="reservedateTextBox" readonly="readonly" id="reservedateTextBox">` but when I tried using the asp:TextBox I cant pass the selected value from date picker to the textbox. So where did I go wrong? Is there a way to program the date picker link to call the popup calendar? (its in Java btw)

Help would be much appreciated! 
Thanks in advance.
Error: 'this._targetEl.value.length' is null or not an object ASP.NET
===

I'm trying to bind the selected value from the date picker to an asp textbox but I have this Error: 'this._targetEl.value.length' is null or not an object. 

Here's the code: 

    <InsertItemTemplate>
        Book Title:
        <asp:DropDownList ID="DropDownList1" runat="server" 
            DataSourceID="booktitleDataSource" DataTextField="booktitle" 
            DataValueField="bookid" SelectedValue='<%# Bind("bookid", "{0}") %>'>
        </asp:DropDownList>
        <asp:SqlDataSource ID="booktitleDataSource" runat="server" 
            ConnectionString="<%$ ConnectionStrings:LibrarySystemConnectionString %>" 
            SelectCommand="SELECT [bookid], [booktitle] FROM [TblBooks]">
        </asp:SqlDataSource>
        <br />
        
        Employee PIN:
        <asp:TextBox ID="employeeidTextBox" runat="server" 
            Text='<%# Bind("employeeid") %>' />
        <br />
        
        Department:
        <asp:TextBox ID="departmentTextBox" runat="server" 
            Text='<%# Bind("department") %>' />
        <br />
        
        Date borrowed:
        <asp:TextBox ID="dateborrowedTextBox" runat="server" Text='<%# Bind("dateborrowed") %>' /> 
    <%--<input type="text" name="reservedateTextBox" readonly="readonly" id="reservedateTextBox">--%> 
        <a href="#" onclick="cdp1.showCalendar(this, 'dateborrowedTextBox'); return false;">Date Picker</a>       
        <br />
        
        <asp:Button ID="InsertButton" runat="server" CausesValidation="True" 
            CommandName="Insert" Text="Insert" />
        <asp:Button ID="InsertCancelButton" runat="server" CausesValidation="False" 
            CommandName="Cancel" Text="Cancel" />
    </InsertItemTemplate>

It was working when I 
tried using `<input type="text" name="reservedateTextBox" readonly="readonly" id="reservedateTextBox">` but when I tried using the asp:TextBox I cant pass the selected value from date picker to the textbox. So where did I go wrong? Is there a way to program the date picker link to call the popup calendar? (its in Java btw)

Help would be much appreciated! 
Thanks in advance.