[BUGFIX] Remove warnings in ImportExportController.php

This patch adds cleanup to prevent warnings when accessing the
import/export module.

Resolves: #58534
Releases: 6.3, 6.2
Change-Id: I1e1afbea07b0c8e8c4bc29d25739882907c79b4b
Reviewed-on: https://review.typo3.org/29881
Reviewed-by: Susanne Moog
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Markus Klein
Tested-by: Markus Klein