Steps to reproduce this issue:
Create a new class in your Wiki (i.e. MySpace.MyClass) and add a TextArea property (i.e. textAreaProperty) to it. Set the "Editor" to "PureText" (other editors will work) Then create a page and add an object of the class' type to it. Set this code as the page's content:
After that, save the page and try to go into / reload the WYSIWYG edit mode. An exception will be thrown, see Stacktrace.txt.