Running into problems when trying to add a node that exists in NPM to NPM without removing and re-adding?

Every time I try to add a node to NCM that already exists in NPM by going to Settings > NCM Settings > Add or Manage Nodes.

I can see

Total Nodes: 0

Nodes Managed By NCM:0

In the left hand pane where I should be able to group nodes by a property I can see "Loading..." which just carries on indefinitely and the page never loads up? Towards the end of the page I can see an error...

Object reference not set to an instance of an object.

   at NodeMapping.GetValuesAndCountForProperty(String property)

Looks like some sort of a SQL error to me. Anybody come across this before?

TIA