Textbox object is not able to create using find control

Question

In my code there is one gridview1 . While clicking on edit button getting error Object reference not set to an instance of an object. all my columns are template fields.id is the datakey. and I am able to fetch the datas from database too. What is wrong here. ?

  protected void Edit_Button_Click(object sender, EventArgs e)
    {
        GridViewRow gr = (GridViewRow)((Button)sender).NamingContainer;
        int id = Convert.ToInt32(GridView1.DataKeys[gr.RowIndex].Value);

        cmd = new SqlCommand("select * from students where id = '"+id+"'",con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        da.Fill(dt);

        TextBox name = (TextBox)GridView1.Rows[gr.RowIndex].FindControl("name_TextBox");



        name.Text = "bhavin";
}

and this is my aspx page.

<asp:TemplateField HeaderText="Name">
                <EditItemTemplate>
                    <asp:TextBox ID="name_TextBox" runat="server" Text='<%# Eval("st_name") %>'></asp:TextBox>
                </EditItemTemplate>
                <ItemTemplate>
                    <asp:Label ID="Label3" runat="server" Text='<%# Eval("st_name") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>

.
.
.
.
<asp:TemplateField HeaderText="Action">
                <EditItemTemplate>
                    <asp:Button ID="Button3" runat="server" Text="Update" />
                    <asp:Button ID="Button4" runat="server" Text="Cancel" />
                </EditItemTemplate>
                <ItemTemplate>
                    <asp:Button ID="Edit_Button" runat="server" Text="Edit" OnClick="Edit_Button_Click" />
                    <asp:Button ID="Button2" runat="server" Text="Delete" />
                </ItemTemplate>
            </asp:TemplateField>

Answer 1

Without your client side code (html) it is a bit hard.

But as you said you use templated columns.

Didn't you forget to place your textbox with ID="name_TextBox"in an EditItemTemplate?

like that :

   <EditItemTemplate> 
    <asp:TextBox id="name_TextBox"><asp:TextBox>
   </EditItemTemplate>

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UPDATED ANSWER

The problem is you can't access control in Gridview_RowEditing. Controls will be rendered after this event (if my memory is still reliable : such a time I moved to MVC). If you manage this event, you have to bind gridview, ex :

 protected void YourGridView_PageIndexChanging(object sender, GridViewPageEventArgs e)
 {
   //you tell Gridview which row gonna be editing : 
   YourGridView.PageIndex = e.NewPageIndex;
   //then you Bind data to the GridView (as you bind data as you did on load for instance)
   YourGridView.DataSource = YourDataSetOrDataTable
   YourGridView.DataBind();
 }

If you want to access a control, you can use gridView_rowDataBound() event instead.