Since ScEditUtils::GetString() requires EditEngine, which is probably not
completely thread-safe.

Change-Id: Ie3d64f56b76a5861fa4b0a1f9cb683cd26806b35
Reviewed-on: https://gerrit.libreoffice.org/54766Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>