balance: Handle null balances correctly

balance.py

@@ -10,12 +10,15 @@ from os import path
 from orders import list_designated_accounts
 
 from beancount import loader
-from beancount.core import data, inventory
+from beancount.core import amount, data, inventory
 
 
 def units(values):
     assert len(values) == 1
-    return values[0].get_only_position().units
+    position = values[0].get_only_position()
+    if position is None:
+        return amount.from_string('0.00 EUR')
+    return position.units
 
 
 def cumulative_balances_by_year(entries, accounts, unpack=False):