I am not sure why you are receiving the error in question, but in a simplified project the code that you have provided does not throw an exception.
Nevertheless, if you need to validate the RadMaskedTextBox
with a RegularExpressionValidator you could try to include the literals as well. In the context of your example, you could try to to include the "-" in the expression. Another option is to use CustomValidator and handle the validation manually.
On a side note, since the RadMaskedTextBox will validate the expression itself, you can use simple RequieredFieldValidator.
As for the exception, if the problem persists, please open regular support ticket with sample, runnable project attached, so we can test it locally.
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