fix

src/customer/admin.py

@@ -47,8 +47,11 @@ class LicenseAdmin(admin.ModelAdmin):
     license_owner.short_description = 'Компания'
 
     def balance_days(self, obj):
-        delta = obj.date_to - timezone.now().date()
-        return delta.days
+        try:
+            delta = obj.date_to - timezone.now().date()
+            return delta.days
+        except AttributeError:
+            return '-'
 
     balance_days.short_description = 'Остаток дней'