fdo#72163: fix safer way

Do not dispose m_xComposer, might still be used by our m_pCache.

Change-Id: I6540c035c9159017c694b36e676721ec3e42db51
Reviewed-on: https://gerrit.libreoffice.org/7094
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>