NPE happens here
https://github.com/h2oai/h2o-3/blob/02b183e7d6d7c83301486ca2fd0ce15b61294379/h2o-core/src/main/java/water/H2ONode.java#L417
when calling
hc.as_h2o_frame() (from Sparkling Water 2.3.19, connected to h2o external backend cluster).
Same code worked fine before upgrade (from SW 2.3.13).
See full exception here -