How to uncheck the checkbox within the grid view template field?

I have two check boxes with the GridView TemplateField. I want to uncheck the checked boxes after submission. My gridview

<asp:GridView ID="GridView1" runat="server" HorizontalAlign="Center" DataKeyNames="ShiftID" 
Width="177px" onrowdatabound="GridView1_RowDataBound1">
<Columns>
   <asp:TemplateField>
     <ItemTemplate>
       <asp:CheckBox ID="ChbGrid" runat="server" 
          oncheckedchanged="ChbGrid_CheckedChanged" />
           </ItemTemplate>
           <HeaderTemplate>
            <asp:CheckBox ID="ChbGridHead" runat="server"  AutoPostBack="True" 
          Font-Bold="True" oncheckedchanged="ChbGridHead_CheckedChanged" />
      </HeaderTemplate>
   </asp:TemplateField>
</Columns>
</asp:GridView>

I tried in below mentioned methode

 public void checkboxclear()
    {
        foreach (GridViewRow row in GridView1.Rows)
        {
            CheckBox chkrow = (CheckBox)row.FindControl("ChbGrid");
            if(chkrow.Checked==true)
            {
                chkrow.Checked = false;//it works 
            }
            else 
            {
                CheckBox chkrow1 = (CheckBox)row.FindControl("ChbGridHead");
                if (chkrow1.Checked == true)
                {
                    chkrow1.Checked = false;//it shows error like "Object reference not set to instance of an object"
                }
            }  
        }  

How can I improve my code to solve this issue? Why I am unable to call these check boxes inside the aspx.cs page