Could not convert JDOM document into string... Root element not set

In some cases, viewing the XML content of a page will result in the following error message:

Could not convert JDOM document into string: Exception outputting Document: Root element not set

To resolve this, try changing the Template content from:

<system-region name="DEFAULT"/>

to:

<xml>
    <system-region name="DEFAULT"/>
</xml>

This will ensure that the content within the DEFAULT region always has a root element.