We need to check read only mode similar to SidebarController::CreatePanels()
method. Otherwise SfxUnoPanels::getByName() creates an invalid panel.

Reviewed-on: https://gerrit.libreoffice.org/47116Tested-by: Xisco Faulí <xiscofauli@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
(cherry picked from commit 23a83639)

Change-Id: Ib7801b81c95f3f505a06c00f749ba4ed5809bfe0
Reviewed-on: https://gerrit.libreoffice.org/47138Tested-by: Xisco Faulí <xiscofauli@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>