[fixed] Preview doesn't work after template change

[Rolf]

SVN Rev. 7371

1. Open an excisting page
2. Open tab Preview -> Preview screen shows
3. Open tab Options and change to another template
4. Open tab Preview -> Preview screen stays blank.
5. Save page, still no preview
6. Go back to listcontent
7. Open page again and preview works

[Rolf]

fixed in rev. 7378