fix: add check to row to prevent error

src/fava_envelope/modules/beancount_envelope.py

@@ -151,7 +151,7 @@ def envelope_tables(self):
         )
         for row in rows[1]:
             if any(regexp.match(row[0]) for regexp in self.budget_accounts):
-                if row[1] is not None:
+                if len(row) > 1 and row[1] is not None:
                     starting_balance += row[1]
         self.income_df[months[0]]["Avail Income"] += starting_balance