tdf#150187: Crash in Presentation Styles - Background objects - Modify

Regression from e85b482df3fa01e5a2b549df63db96cc49f1ca78
(use SfxItemSet::GetItemIfSet in cui)

Change-Id: I74889f21f2bda248946269b0caf5ddc629002ae0
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/137580
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>
(cherry picked from commit 740cdbb68b3357a5e3894f742c5804c5b8a21e02)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/137533
Tested-by: Jenkins
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>
diff --git a/cui/source/tabpages/numpages.cxx b/cui/source/tabpages/numpages.cxx
index 215e59b..050e8c7 100644
--- a/cui/source/tabpages/numpages.cxx
+++ b/cui/source/tabpages/numpages.cxx
@@ -418,6 +418,12 @@ void  SvxBulletPickTabPage::Reset( const SfxItemSet* rSet )
    {
        nNumItemId = rSet->GetPool()->GetWhich(SID_ATTR_NUMBERING_RULE);
        pItem = rSet->GetItemIfSet(nNumItemId, false);

        if( !pItem )
        {
            pItem = & rSet->Get( nNumItemId );
        }

    }
    pSaveNum.reset( new SvxNumRule(pItem->GetNumRule()) );