This is apparently caused by the changes made in https://issues.jboss.org/browse/WELD-992
Methods added through AfterBeanDiscovery.addObserverMethod() previously didn't fire an event, but now they do. And the event actually has the annotatedMethod set to null (see line 45 at https://github.com/weld/core/blob/master/impl/src/main/java/org/jboss/weld/bootstrap/events/ProcessObserverMethodImpl.java#L45 )
Basically, Solder needs to handle the null, since it looks like there's no way to get the AnnotatedMethod from the ObserverMethod passed to addObserverMethod (at least for now... need to see what CDI1.1 says about this).