Handle scenarios flagged by type checking where `nodeFromXPath` may
return `null` or a non-Element node. Previously it could never return
`null` but would throw an exception if the node was not found. In the
new code a check is needed.

The case where `nodeFromXPath` returned a non-Element node would never
happen with XPaths generated by the client but could happen if an
annotation was created via the API with a non-simple XPath.