The `onthrow` option of the JDWP-agent causes the agent to only initialize the debugging session after a specificied exception has been thrown. The agent then sends an exception event with the exception that triggered the session start. The sent exception has a null pointer for all its properties:

```
Exception event:
    thread: 0x0
    clazz: 0x0
    method: 0x0
    location: 0
    object: 0x0
    catch_clazz: 0x0
    catch_method: 0x0
    catch_location: 0
```

This in turn causes jdb to throw an internal NullPointerException:

```
Exception in thread "event-handler" java.lang.NullPointerException: Cannot invoke "com.sun.jdi.ObjectReference.referenceType()" because the return value of "com.sun.jdi.event.ExceptionEvent.exception()" is null
        at jdk.jdi/com.sun.tools.example.debug.tty.TTY.exceptionEvent(TTY.java:171)
        at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.exceptionEvent(EventHandler.java:295)
        at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.handleEvent(EventHandler.java:133)
        at jdk.jdi/com.sun.tools.example.debug.tty.EventHandler.run(EventHandler.java:78)
        at java.base/java.lang.Thread.run(Thread.java:1583)
```