Resolves: tdf#104153 crash on drag and drop pivot table field

now though sometimes it doesn't get removed at all, so a band aid
for the moment

Change-Id: I6ccbbe51fe9250af0f85c30a9d253269a18df457

sc/source/ui/dbgui/PivotLayoutTreeListLabel.cxx

@@ -59,7 +59,7 @@ void ScPivotLayoutTreeListLabel::FillLabelFields(ScDPLabelDataVector& rLabelVect
 
 void ScPivotLayoutTreeListLabel::InsertEntryForSourceTarget(SvTreeListEntry* /*pSource*/, SvTreeListEntry* /*pTarget*/)
 {
-    if(mpParent->mpPreviouslyFocusedListBox.get() != this)
+    if (mpParent->mpPreviouslyFocusedListBox && mpParent->mpPreviouslyFocusedListBox.get() != this)
         mpParent->mpPreviouslyFocusedListBox->RemoveSelection();
 }