Fix segmentation fault in autotest ViewerGrantleeThemeSupportTest

Summary:
ViewerGrantleeThemeSupportTest would complete its tests successfully,
print "Release of profile requested but WebEnginePage still not deleted.
Expect troubles !", and crash. It just needed to delete the message viewer,
which deletes its child WebEnginePage.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D22938