Fixes florent37#148

The crash was caused because of the options secondDateAfterFirst which was added in the commit db112c9
The issue was only reproduced when the flag secondDateAfterFirst was set to true and the WheelDayOfMonthPicker view was visible.
The listener addOnDateChangedListener was updating the second picker with the minimum date whereas when the WheelDayOfMonthPicker's visibility was gone, the picker had no items resulting in a crash in getItem() method of the WheelPicker adapter.

singledateandtimepicker/src/main/java/com/github/florent37/singledateandtimepicker/widget/WheelPicker.java

@@ -1068,7 +1068,7 @@ public int getItemCount() {
         @Override
         public V getItem(int position) {
             final int itemCount = getItemCount();
-            return data.get((position + itemCount) % itemCount);
+            return itemCount != 0 ? data.get((position + itemCount) % itemCount) : null;
         }
 
         @Override