The JoinProcessor always expects identification properties on all nodes coming from sources. Since some connectors do not have identification properties, this causes a NullPointerException when accumulating the identification properties:

java.lang.NullPointerException
at org.jboss.dna.graph.connector.federation.JoinRequestProcessor.process(JoinRequestProcessor.java:272)
at org.jboss.dna.graph.request.processor.RequestProcessor.process(RequestProcessor.java:235)
at org.jboss.dna.graph.connector.federation.JoinRequestProcessor.process(JoinRequestProcessor.java:164)
at org.jboss.dna.graph.connector.federation.FederatedRepositoryConnection.execute(FederatedRepositoryConnection.java:220)
at org.jboss.dna.graph.connector.RepositoryConnectionPool$ConnectionWrapper.execute(RepositoryConnectionPool.java:991)
at org.jboss.dna.graph.Graph.execute(Graph.java:243)
at org.jboss.dna.graph.Graph$5.process(Graph.java:186)
at org.jboss.dna.graph.request.RequestBuilder.readNode(RequestBuilder.java:139)
at org.jboss.dna.graph.Graph.getNodeAt(Graph.java:1843)
at org.jboss.dna.graph.Graph.getNodeAt(Graph.java:1853)

The problem is this snippet:

// Accumulate the identification properties ...
for (Property propertyInSource : readFromSource.getActualLocationOfNode().getIdProperties())

since the JavaDoc for getIdProperties() says that it should return null if there are no properties. This code should be wrapped with a check to see if the Location has identification properties.