Notes from reproduction testing (in case anyone else sees the same error at step 4 below):

1. Created a new Tree, e.g. Test14062.
2. Added a Zero Page Login Collector.  Set the 'Has Credentials' outcome to go to a DataStore Decision Node.  Then set the relevant outcomes for this go to success and failure.
3. For the Zero Page Login Collector node 'No Credentials' outcome, added a Failure URL and set this to be http://www.example.com
4. Tested the tree using e.g. https://openam.amtest2.com:8443/access?service=Test14062.  This resulted in the text 'Loading...' appearing top left in the browser.  In the network tab, the call to https://openam.amtest2.com:8443/access/json/realms/root/authenticate?service=Test14062&authIndexType=service&authIndexValue=Test14062 had resulted in an error 500.  IN the CoreSystem debug logs the cause was:
    

   Caused by: java.lang.NullPointerException
           at org.forgerock.openam.auth.trees.engine.AuthTree.getNextNodeId(AuthTree.java:312)
           at org.forgerock.openam.auth.trees.engine.AuthTreeExecutor.process(AuthTreeExecutor.java:131)
           at org.forgerock.openam.auth.trees.engine.AuthTreeExecutor.process(AuthTreeExecutor.java:142)
           at org.forgerock.openam.core.rest.authn.trees.AuthTrees.processTree(AuthTrees.java:389)
           at org.forgerock.openam.core.rest.authn.trees.AuthTrees.evaluateTreeAndProcessResult(AuthTrees.java:245)
           at org.forgerock.openam.core.rest.authn.trees.AuthTrees.invokeTree(AuthTrees.java:237)
           at org.forgerock.openam.core.rest.authn.RestAuthenticationHandler.authenticate(RestAuthenticationHandler.java:207)
           at org.forgerock.openam.core.rest.authn.http.AuthenticationServiceV1.authenticate(AuthenticationServiceV1.java:158)
   

1. Set the outcome of the Failure URL node to be Failure.  Then repeated step 4 above.  Now I see the current behaviour described in this JIRA.