Both http and bolt dialects run into NPE if a "commit" of modified managed entities happens without an active transaction.
For Bolt, the tx object accessed in BoltNeo4jEntityQueries (line 57) is null. For Http the tx object accessed in HttpNeo4jResourceLocalTransactionCoordinator (line 343) is null as well.
You can use jpa-unit neo4j TransactionTest to reproduce the issue. Change the settings in the persistence.xml of the corresponding project to neo4j_http/neo4_bolt to run tests using these different protocols. To run the tests you'll need a running Neo4j instance.