[BUGFIX] Can't use gridelements functionality in Page module

Since TYPO3 6.2 it's not possible to
 use the extended functionality of gridelements:

Drag'n'drop content elements
Clicking the "new element" doesn't bring down the content elements picker - nothing happens
The only javascript error I can pick up from my console is this

Uncaught TypeError: Cannot read property 'parentNode' of null

Change-Id: Ibec47f1d6398f4b8f6eb467a6ef7e235df8c2994
Resolves: #53415
Reviewed-on: https://review.typo3.org/25395
Reviewed-by: Soren Malling
Tested-by: Soren Malling
Reviewed-by: Jo Hasenau
Tested-by: Jo Hasenau