[BUGFIX] Use correct element for `TYPO3.Backend.ContentContainer.getUrl()`

`TYPO3.Backend.ContentContainer.setUrl()` writes the URL into a different
element than `TYPO3.Backend.ContentContainer.getUrl()` tries to read,
resulting in a wrong URL.

This patch changes the behavior of `getUrl()` to read the URL from the
correct element.

Resolves: #78229
Related: #52877
Releases: master
Change-Id: Ie70c89d4a195fd37b18c11a86d2a672760430621
Reviewed-on: https://review.typo3.org/50204


Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>