Hi Martin,
Thanks for the response. I am completely baffled by what is going on. I created a blank project and everything works just fine. So I started copying all of my existing project over to the one to try and see what would cause it to break. It never broke. So then I figured it must be some project configuration issue. So I opened each project file in notepad and compared line by line and everything matches.
No matter what I do, I can't get the original project to work.
My OCD isn't letting me drop this even though the new project works (I'm concerned the new project will work temporarily and then I'll do something to cause the same error again). Do you have any advice as to how I can debug this? I've attached the VS debugger to the XAML Designer and found that whenever I modify the Style attribute of one of the controls in question, a System.ArgumentNullException is thrown. The null value causing the exception is the resourceKey for the FrameworkElement (which is a TextBox in this case).
I hope this helps.