Check if data is defined

Otherwise when adding a user as subadmin this would throw the following JS error:

```
users.js?v=bc2aae7…:500 Uncaught TypeError: Cannot read property 'message' of undefined(…)(anonymous function) @ users.js?v=bc2aae7…:500j @ jquery.js:3099fireWith @ jquery.js:3211x @ jquery.js:8264(anonymous function) @ jquery.js:8605
```

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>