tdf#117426 don't crash is infobar is disposed in preview view

Change-Id: Iff2dece2ea5f65c5d0896af888e05703ae709c65
Reviewed-on: https://gerrit.libreoffice.org/59512
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sfx2/source/doc/objserv.cxx

@@ -1079,7 +1079,7 @@ void SfxObjectShell::GetState_Impl(SfxItemSet &rSet)
                         if ( !sMessage.isEmpty() )
                         {
                             auto pInfoBar = pFrame->AppendInfoBar("signature", sMessage, aInfoBarType);
-                            if (pInfoBar == nullptr)
+                            if (pInfoBar == nullptr || pInfoBar->IsDisposed())
                                 return;
                             VclPtrInstance<PushButton> xBtn(&(pFrame->GetWindow()));
                             xBtn->SetText(SfxResId(STR_SIGNATURE_SHOW));