Table::dispose is being executed, so its not IsDisposed yet, but this
a11y code gets called during the dispose and the children have been cleared
at this point

Change-Id: I20aa9b15b1c5c3da91af58ac61f59fcf52619d58
(cherry picked from commit 4bb2d015)
Reviewed-on: https://gerrit.libreoffice.org/16825Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>