Difference between Wikidata Graph Builder and WDQS

[jaygray0919]

I have this Wikidata query:

SELECT ?root_name ?parent_name ?child_name WHERE
{
 VALUES ?root_name {"Brassica rapa"}
 ?root wdt:P225 ?root_name .
 ?child wdt:P171+ ?root .
 ?child wdt:P171 ?parent .
 ?child wdt:P225 ?child_name .
 ?parent wdt:P225 ?parent_name .
 SERVICE wikibase:label {bd:serviceParam wikibase:language "en" }
}

serialized here:
https://query.wikidata.org/#SELECT%20%3Froot_name%20%3Fparent_name%20%3Fchild_name%20WHERE%0A%7B%0A%20VALUES%20%3Froot_name%20%7B%22Brassica%20rapa%22%7D%0A%20%3Froot%20wdt%3AP225%20%3Froot_name%20.%0A%20%3Fchild%20wdt%3AP171%2B%20%3Froot%20.%0A%20%3Fchild%20wdt%3AP171%20%3Fparent%20.%0A%20%3Fchild%20wdt%3AP225%20%3Fchild_name%20.%0A%20%3Fparent%20wdt%3AP225%20%3Fparent_name%20.%0A%20SERVICE%20wikibase%3Alabel%20%7Bbd%3AserviceParam%20wikibase%3Alanguage%20%22en%22%20%7D%0A%7D

When using
display results: treemap
WD produces a structured hierarchy

Similarly
display results: dimensions
plots the tree

When I use the above query on Wikidata Graph Builder, I see the error message
Unable to load graph data
me.item is undefined

Is this user-error?
Is there a working WGB query that will display the child items for the parent root_name?

Thanks for advice here.
/jay