[symptoms]
You created a new site and then tried to set a property in the "All Properties" dialog. A dialog came up saying:
There was an error updating the objects.
Object reference set to an instance of an object.
The properties are persisted despite the error message. You have to click "Cancel" to get out of the dialog.
[workarounds]
1) Close IIS Manager and reopen.
2) Tolerate the error dialog - i.e. click OK on the dialog, dismiss the error, and then click Cancel to get out of the dialog.