Ignore removed referenceable node types

Hi there

If you choose a node type in the field settings and afterwards you delete this node type there is an error on the manage field page as well as on the node add/edit page:

Notice: Trying to get property of non-object in references_dialog_node_reference_add_link()

A quick solution is to check if the node type could be loaded (see attached patch). A better solution would be to remove the node type from the field settings, if it doesn't exists.