如何在按钮点击时更改内容占位符？

Question

当我调试时，我收到一条错误消息，显示“对象引用未设置为对象的实例”。

这是我的代码...............

protected void Page_Load(object sender, EventArgs e)
{
    ContentPlaceHolder cph1 = (ContentPlaceHolder)this.Page.Master.FindControl("ContentPlaceHolder1");
    ContentPlaceHolder cph2 = (ContentPlaceHolder)this.Page.Master.FindControl("ContentPlaceHolder2");
    cph1.Visible = true;
    cph2.Visible = false;
}
protected void Button1_Click(object sender, EventArgs e)
{
    ContentPlaceHolder cph1 = (ContentPlaceHolder)this.Page.Master.FindControl("ContentPlaceHolder1");
    ContentPlaceHolder cph2 = (ContentPlaceHolder)this.Page.Master.FindControl("ContentPlaceHolder2");
    cph1.Visible = true;
    cph2.Visible = false;

    String ltype=DropDownList1.SelectedValue.ToString();
    momconnect obj = new momconnect();
    obj.getconnect();
    SqlCommand cmd1 = new SqlCommand("Select Levdef_id from Tbl_leave_define where Levtype='"+ltype+"'", obj.conn);
    SqlDataReader dr = cmd1.ExecuteReader();
    dr.Read();
    Session["defid"]=dr["Levdef_id"].ToString();
    dr.Close();

    SqlCommand cmd = new SqlCommand("sp_leave_request", obj.conn);
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("@flag", 1);
    cmd.Parameters.AddWithValue("@Staff_id", Session["staffid"].ToString());
    cmd.Parameters.AddWithValue("@Levtype",ltype);
    cmd.Parameters.AddWithValue("@Reason",TextBox1.Text);
    cmd.Parameters.AddWithValue("@Num", TextBox2.Text);
    cmd.Parameters.AddWithValue("@Frmdate", TextBox3.Text);
    cmd.Parameters.AddWithValue("@Todate",TextBox4.Text);
    cmd.Parameters.AddWithValue("@Levdef_id", Session["defid"].ToString());
    cmd.ExecuteNonQuery();

    cph1.Visible = false;
    cph2.Visible = true;
    Label1.Text = "Leave request sent. Wait for approval";



}