DesignTime Error Wpf ValidatesOnTargetUpdated NullReferenceException

I have the same question as in the link below, but the answers where not satisfying for me.

ValidationRule ValidatesOnTargetUpdated NullReferenceException at Design Time

XAML

<TextBox x:Name="MyTextBox">
  <TextBox.Text>
    <Binding Path="MyText">
    </Binding>
  </TextBox.Text>
</TextBox>

Code-Behind

Binding binding = BindingOperations.GetBinding(MyTextBox, TextBox.TextProperty);
     binding?.ValidationRules.Add(new MyValidationRule()
      {
        ValidatesOnTargetUpdated = true
      });

It works with this section in code behind. But I am looking for a only XAML solution, because it is more intuitive and for me better maintainable. Does anybody have an idea?

I found a "solution" (or more a workaround) on my own. The designer works for me if I change ValidatesOnTargetUpdated to ValidationStep with the wished value.

Attention: For example, the TextBox behaves differently if you put in ValidationStep="UpdatedValue" and ValidationStep="ConvertedProposedValue". More information on ValidationRule with ValidationStep="UpdatedValue" is called with BindingExpression instead of updated value.

However, I could not find why ValidatesOnTargetUpdated did not work.