Radio button Checked Event throws null reference exception

here is some more simpler approach

   private void rdiobtn_Checked(object sender, RoutedEventArgs e)
 {
//include here some mothod to clear the previous state of secondRadioButton
//like in asp.net your can include secondRadioButton.ClearSelection();
//same way in window application also there must be some method, what you can in the intelsence
   secondRadioButton.IsChecked =       !firstRadioButton.IsChecked;
}

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Please mark as answer, if you find this post helpful. Thanks Deepak Kalra