I've checked the code of this addon. And if you open there FeedbackOverlay.xaml.cs you will find that Message property lacks get accessor:
public string Message
{
// get { //something } - is not here
set
{
// some code
}
}
But in this case (as this property is used only to define a message - not get it) the get accessor isn't needed. So the exception you get is probably only from Visual Studio and it informs that there can be a problem if you had tried to get value of Message.
So unless you try to get the value of Message - for example:
string myMessage = FeedbackOverlay.Message;
there should be no problem (VS should also mark this code above). There can be also a problem if you try to get this value in xaml (for example using Binding).
I also think that it would be nice if you had informed the developer (you will probably find his e-mail easily on Github or somewhere in the project) that you encountered such an issue.
Hope this helps.