How to Control that wich of the checkboxes checked

Hi, this is my ASPX Page

<asp:Repeater runat="server" ID="rptChilds">
    <ItemTemplate>
        <asp:CheckBox ID="chkc" runat="server" Text="" />
    <br />
    </ItemTemplate>
</asp:Repeater>

i want to check that wich of the checkboxes check, for example by code as this

for (j = 0; j < NumberOfCheckboxes; j++)
                {
                    CheckBox chk = (CheckBox)(rptChilds).FindControl("chkc");
                    if (chk != null)
                    {
                        if (chk.Checked)
                            //do something here
                    }
                    else
                    {
                    //error
                    }
                }

and this is the result:

Server Error in '/Project' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 


Line 250:                    {
Line 251:                        CheckBox chk = (CheckBox)(rptChilds).FindControl("chkc");
Line 252:                        if (chk.Checked)
Line 253:                        {
Line 254:                            //your code here

Source File: f:\Documents\    \Projects\MahdZNU\Project\Pages\Image Gallery\showImages.aspx.cs    Line: 252 

Stack Trace: 


[NullReferenceException: Object reference not set to an instance of an object.]
   Pages_Image_Gallery_showImages.insertImageIntoGallery(Object sender, EventArgs e) in f:\Documents\                  \Projects\MahdZNU\Project\Pages\Image Gallery\showImages.aspx.cs:252
   System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e) +116
   System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument) +101
   System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +9642898
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1724

Tags: ASP.NET, Checkbox