[BUGFIX] Fix of Close-button in flash messages

The close button did not work because hide() was called on
a HTMLElement instead of an Ext.Element.

Fixes: #46373
Related: #31368
Releases: 6.1, 6.0, 4.7
Change-Id: I70c65a82683ece39de4ed043a916b0521aabafa1
Reviewed-on: https://review.typo3.org/19465
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn