Closed Issue: maskedtextbox crashs when multibinding mask [21456]

Hi,

Here's the XAML code
```
<DockPanel Height="50">
<StackPanel>
<RadioButton Content="Físico" x:Name="rdbFisico" GroupName="tipoPessoa">
<RadioButton.Style>
<Style TargetType="RadioButton">
<Style.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Tag" Value="000.000.000-00"/>
</Trigger>
</Style.Triggers>
</Style>
</RadioButton.Style>
</RadioButton>
<RadioButton Content="Jurídico" x:Name="rdbJuridico" GroupName="tipoPessoa">
<RadioButton.Style>
<Style TargetType="RadioButton">
<Style.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Tag" Value="00.000.000/0000-00"/>
</Trigger>
</Style.Triggers>
</Style>
</RadioButton.Style>
</RadioButton>
</StackPanel>
<eXwpf:MaskedTextBox x:Name="Documento" Height="20">
<eXwpf:MaskedTextBox.Mask>
<MultiBinding Converter="{StaticResource TipoPessoaRadioCheckMask}">
<Binding ElementName="rdbJuridico" Path="Tag" FallbackValue="00"/>
<Binding ElementName="rdbFisico" Path="Tag" FallbackValue="00"/>
</MultiBinding>
</eXwpf:MaskedTextBox.Mask>
</eXwpf:MaskedTextBox>
</DockPanel>
```
Converter code
```

public class TipoPessoaRadioCheckMask : IMultiValueConverter
{
#region IMultiValueConverter Members

public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
object rval = string.Empty;
foreach (object oneObject in values)
{
if (oneObject != null)
{
rval = oneObject.ToString();
break;
}
}
return rval;
}

public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}

#endregion
```