handle MultipleObjectsReturned for form data

We also need to handle MultipleObjectsReturned as we allow nullable `display_key` in table. So when display_key url param is empty, above exception is raised.

We could also do, 

```
if display_key:
        data = FormModelData.objects.get(display_key=display_key)
else:
        data = None
```
Let me know which one you prefer.