Replace assert with early return, in case the collection was deleted meanwhile.

Another comment in the same file says
 // Do not assert on invalid collections, since a collection may be deleted
 // in the meantime and deleted collections are invalid.
Therefore it looks like this is expected.

src/widgets/collectionstatisticsdelegate.cpp

@@ -208,7 +208,7 @@ void CollectionStatisticsDelegate::paint(QPainter *painter,
     // in initStyleOption(), which gets called by QStyledItemDelegate::paint().
     QStyledItemDelegate::paint(painter, option, index);
 
-    // No, we retrieve the correct style option by calling intiStyleOption from
+    // Now, we retrieve the correct style option by calling initStyleOption from
     // the superclass.
     QStyleOptionViewItemV4 option4 = option;
     QStyledItemDelegate::initStyleOption(&option4, index);
@@ -235,10 +235,9 @@ void CollectionStatisticsDelegate::paint(QPainter *painter,
 
     if (!collection.isValid()) {
         qCCritical(AKONADIWIDGETS_LOG) << "Invalid collection: " << collection;
+        return;
     }
 
-    Q_ASSERT(collection.isValid());   // TODO: I seem to hit this when removing a duplicated "Personal Contacts" or "Personal Calendar"
-
     CollectionStatistics statistics = collection.statistics();
 
     qint64 unreadCount = qMax(0LL, statistics.unreadCount());