The second parameter of bs.LinkNavigator.Navigate method is source that cannot be null. Try this:
private void addGameButton_Click(object sender, RoutedEventArgs e)
{
BBCodeBlock bs = new BBCodeBlock();
try
{
bs.LinkNavigator.Navigate(new Uri("/Pages/AddGame.xaml", UriKind.Relative), this);
}
catch (Exception error)
{
ModernDialog.ShowMessage(error.Message, FirstFloor.ModernUI.Resources.NavigationFailed, MessageBoxButton.OK);
}
}