Fix invoice method computation for task without project

issue3175
review819002
(transplanted from 87f0ad1c2062f47b89e7baa895e1341cafe95f79)

work.py

@@ -88,8 +88,10 @@ class Work:
     def get_invoice_method(self, name):
         if self.type == 'project':
             return self.project_invoice_method
-        else:
+        elif self.parent:
             return self.parent.invoice_method
+        else:
+            return 'manual'
 
     @staticmethod
     def default_invoiced_hours():