[BUGFIX] Fix warnings in em on tab Maintenance

One variable is used without checking if it is set and on the other
place an array key is used twice which is just a mistake of the coder.

Change-Id: I78576e0ef7b8ca0b63ae94f74e2a5dfec659d716
Fixes: #39680
Releases: 4.5, 4.6, 4.7
Reviewed-on: https://review.typo3.org/17724
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn