How to find checkbox control in edittemplste in gridview?

I want to update my database via using gridview. I have a checkbox in gridview but when I try to update I define checkbox but it seems null always. And when I try to run the following code I get systemnullreferenceexception error. How can I fix this error?

This is my aspx.cs part:

        else if (e.CommandName == "UpdateRow") {
            int rowIndex = ((GridViewRow)((LinkButton)e.CommandSource).NamingContainer).RowIndex;
            int ID = Convert.ToInt32(e.CommandArgument);
            DateTime start_time = Convert.ToDateTime(((TextBox)EditGrid.Rows[rowIndex].FindControl("TxtUpdateStart")).Text);
            DateTime finish_time = Convert.ToDateTime(((TextBox)EditGrid.Rows[rowIndex].FindControl("TxtUpdateFinish")).Text);
            CheckBox ck = ((CheckBox)EditGrid.Rows[rowIndex].FindControl("ChkEditOverride"));
            bool finish_override;
            if (ck.Checked)
                finish_override = true;
            else
                finish_override = false;
            settings.UpdateRecord(ID, start_time, finish_time, finish_override);

            EditGrid.EditIndex = -1;
            BindGridViewData();

This is my aspx part:

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