fix focus on product in save and clear cb

grocery_transactions.py

@@ -106,7 +106,7 @@ def _save_and_clear_callback(activity_manager):
     txn = activity_manager.get('transaction')
     activity_manager.app.save(txn.data)
     txn.clear()
-    txn.focus_on_product()
+    txn.focus_on(txn.edit_fields['product'])
     activity_manager.show(txn.update())
 
 args = sys.argv