[BUG] Have better error handling if the `object_id_field` can't be read

[jzonthemtn]

Is your feature request related to a problem?

Refer to #26, if the user gives a value for the object_id_field whose value is an array, the plugin will throw a class cast exception. The plugin should instead handle that gracefully.

[epugh]

@jzonthemtn can this ticket now be closed? Looks like it might be ready...