Unimplemented got thrown for null. Add it to the list so that lists with no items still work properly.

lib/pages/report.dart

@@ -153,6 +153,9 @@ class SummaryCardForTotals extends StatelessWidget {
 
           /* Load page variables based on calculated totals. */
           switch (itemType) {
+            case null:
+              break;
+
             case ItemType.asset:
               _assetTotal = monthlyTotal;
               break;