Erro object reference not set to an instance of an object

[URL=http://imageshack.us][img:8c68e73581]http://img384.imageshack.us/img384/148/errocucj2.jpg[/img:8c68e73581][/URL][URL=http://g.imageshack.us/img384/errocucj2.jpg/1/][img:8c68e73581]http://img384.imageshack.us/img384/errocucj2.jpg/1/w1280.png[/img:8c68e73581][/URL]

Quando eu chamo o formulário filho aparece este erro...