How to force ValidatingModelBase.Validate works on main thread?

[GF-Huang]

I'm using AvalonEdit, in my VM there's a TextDocument, which is used to logically control the TextEditor in the view, so it is not a view/control, so it is not wrong for me to put it in the VM. But the TextDocument not allow access from other thread except the main thread (even if only read, not write). So when I call Validate, it show me an exception.

And I use MvvmValidation as my underlying validation framework.
It will call the lambda when Validate called. So the ApplicationsDocument will accessed from other thread.