I was able to figure out what was causing this error.
There was a bit of code in our application that attempted to locate a resource by a given index. The resource was not found and consequently, the value returned was null.
The null value then populated the UnicodeString property of the TextBox. This did not immediately generate an exception and allowed the method to continue.
My assumption would be that the exception is thrown when the WPF framework attempts to render the frame that contains the null UnicodeString, which explains the lack of any application code in the stack trace.