Aspcalendar is not displayed when imagebutton is clicked and also getting error fill this field

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C#

ASP.NET

calendar

I have a input type text field with imagebutton and AspCalendar controls in an asp.net project. When i am clicking the image button then the calendar visibility false in pageload event and also getting the error "Fill out required fields" from top to bottom in between calendar controls appears.

What I have tried:

Html Code:

<%-- Start Date --%>
<div class="form-group row">
    <label for="inputTxtStartDate" class="col-sm-2 col-form-label">Start Date</label>
    <div class="col-10">
        <input type="text" id="inputTxtStartDate" name="inputTxtStartDate" class="form-control-sm" runat="server" />
        <asp:ImageButton ID="imgBtnStartDateCalendar" runat="server" ImageUrl="~/Images/Calendar.ico" Width="20px" OnClick="imgBtnStartDateCalendar_Click" CausesValidation="false" />
        <asp:Calendar ID="Calendar1" runat="server" OnSelectionChanged="Calendar1_SelectionChanged" NextPrevFormat="FullMonth" SelectionMode="DayWeekMonth" SelectMonthText="Month" SelectWeekText="Week" ForeColor="WhiteSmoke" DayNameFormat="Full" Font-Names="Book Antiqua" Font-Size="XX-Small">
            <DayHeaderStyle BackColor="DarkOliveGreen" />
            <DayStyle BackColor="DarkKhaki" BorderColor="Khaki" BorderWidth="1" Font-Bold="true" Font-Italic="true" />
            <NextPrevStyle Font-Italic="true" Font-Names="Arial CE" />
            <OtherMonthDayStyle BackColor="SeaGreen" BorderColor="DarkSeaGreen" />
            <SelectedDayStyle BackColor="Green" BorderColor="SpringGreen" />
            <SelectorStyle BackColor="DarkOliveGreen" ForeColor="Snow" Font-Names="Times New Roman Greek" Font-Size="Small" BorderColor="Olive"                                                BorderWidth="1" />
            <TitleStyle BackColor="DarkGreen" Height="35" Font-Size="XX-Small" Font-Names="Courier New Baltic" />
        </asp:Calendar>
    </div>
</div>

Code Behind:

PageLoad Event:

Calendar1.Visible = false;

ImageButton Click Event:

if (Calendar1.Visible == true)
    Calendar1.Visible = false;
else
    Calendar1.Visible = true;