NullReferenceException error, c#

This code generates a few buttons dynamically:

private void makeButtonFor(int id, string caption)
    {
        Button b = new Button();
        {
            var temp = "medButton" + id.ToString();
            b.ID = temp; 
            b.Text = caption;
            var captionButton = (caption.Substring(0, 6) == "Region") ? "RegionButton" : "medCtrButtonStyle";
            b.ControlStyle.CssClass =captionButton;
            myPanel2.Controls.Add(b);
            b.Click += new System.EventHandler(medCtrSubmit_click);

        };
    }

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The values in temp are like "medButton1",  "medButton2", etc.

Question: Why btn is null at line 5 in the code below?

            
if (IsPostBack)
            {
               Button btn = (Button)this.FindControl("medButton2");
               btn.BackColor = System.Drawing.Color.FromArgb(0, 138, 105, 212);
               System.Drawing.Color.FromArgb(16, btn2.BackColor);
            }

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