Error when adding a new site without specifying the property where it's located

[h-m-m]

Steps to recreate:

• Click on the "Manage Sites" link on the main page
• theLog in to the ActiveAdmin console
• Click on the "New Site" button
• Add a name for the site but do not select a property
• Click on "Create Site"

Expected result:
You should see a warning near the "Property" dropdown that a property must be specified.

Observed result:
The application responds with its HTTP 500 error page

[h-m-m]

Additionally, if the list of properties are empty, it would be nice to redirect to the /admin/properties page with an explanation message at the top of that page that you can't add a site without adding a property first. This is a nice to have it's an easy change, but it is a low-priority addition.