Custom Server Controls :: Rendering Controls Via Loop Inside Render

I am creatin a server control where some controls are being rendered inside `protected override void Render(HtmlTextWriter writer)`

writer.AddAttribute(HtmlTextWriterAttribute.Type, "button");
writer.AddAttribute(HtmlTextWriterAttribute.Id, this.UniqueID + "butto_Foo1");
writer.AddAttribute(HtmlTextWriterAttribute.Value, "Button1");
writer.RenderBeginTag(HtmlTextWriterTag.Input);
writer.RenderEndTag(); //button
writer.AddAttribute(HtmlTextWriterAttribute.Type, "button");
writer.AddAttribute(HtmlTextWriterAttribute.Id, this.UniqueID + "butto_Foo2");
writer.AddAttribute(HtmlTextWriterAttribute.Value, "Button2");
writer.RenderBeginTag(HtmlTextWriterTag.Input);
writer.RenderEndTag(); //button

This way buttons are rendered on design view as well. The problem is that number of buttons to render is dynamic based on data in xml file (even if the data was static, the number of button is about 12 and I don't think it is wise to copy paste the same code above 6 times...). I thought by using loop for that:

int i = 1;
foreach (Button button in tb.Items)
{
writer.AddAttribute(HtmlTextWriterAttribute.Type, "button");
writer.AddAttribute(HtmlTextWriterAttribute.Id, this.UniqueID + "butto_Foo" + i);
writer.AddAttribute(HtmlTextWriterAttribute.Value, button.ButtonValue);
writer.RenderBeginTag(HtmlTextWriterTag.Input);
writer.RenderEndTag(); //button
i++;
}

tb is defined in OnLoad; On run time it works, but on design time I get error: > Error rendering control. And control is not being rendered.