Fix reload tree

Reproduce:
1. create your own Module
2. add some Fields to the DCA
3. add a mandatory `pageTree` Widget
4. create a new entry in the backend
5. choose a Page and on closing the modal window you run into a Bad Request

-> When creating a new entry No ID is in the get Parameter. So we have to check if it is `null` and go ahead without creating the `$objModel`

I have set `$arrAttribs['activeRecord'] = null;` to `null`. Not sure if something else is broken in that case. Please check.