When I use rule agent with directory scanner, and the rule that is being scanned contains a bug, then I receive a NPE instead of error message:

java.lang.NullPointerException
at org.drools.agent.FileScanner.readPackage(FileScanner.java:102)
at org.drools.agent.FileScanner.getChangeSet(FileScanner.java:79)
at org.drools.agent.FileScanner.loadPackageChanges(FileScanner.java:57)
at org.drools.agent.DirectoryScanner.loadPackageChanges(DirectoryScanner.java:43)
at org.drools.agent.RuleAgent.checkForChanges(RuleAgent.java:330)
at org.drools.agent.RuleAgent.refreshRuleBase(RuleAgent.java:298)
at org.drools.agent.RuleAgent.configure(RuleAgent.java:284)
at org.drools.agent.RuleAgent.init(RuleAgent.java:208)
at org.drools.agent.RuleAgent.newRuleAgent(RuleAgent.java:176)
at org.drools.agent.RuleAgent.newRuleAgent(RuleAgent.java:164)
... (further entries of stack trace contains my application's lines)

When I looked into code I found that DirectoryScanner object, used by the agent uses FileScanner object to scan each file in a directory. And the file scanner does not have a listener set. That's why the NPE is thrown.