Since 8u20 there are NPEs coming from the JAB code, most likely due to the fix for JDK-8037477.  The NPE happens in many different scenarios but the one that can cause it quicker than the rest is to press Alt+F to bring up the menu system and then left/right arrow between the first pulldown and the second pulldown.  The problem appears to be due to a mismatch between the reported number of accessible children and the actual number of accessible children.  In com.sun.java.accessibility.AccessBridge._getVisibleChild there is this code:

            final AccessibleContext ac2=InvocationUtils.invokeAndWait(new Callable<AccessibleContext>() {
                @Override
                public AccessibleContext call() throws Exception {
                    Accessible a = ac.getAccessibleChild(idx);
                    return a.getAccessibleContext();
                }
            }, ac);

That callable is later called in invokeAndWait this try/catch:

            try {
                SunToolkit.postEvent(targetAppContext, event);
                latch.await();
                T result = wrapper.getResult();
                updateAppContextMap(result, targetAppContext);
                return result;
            } catch (final Exception e) {
                throw new RuntimeException(e);
            }

If there is no accessible for one of the items being iterated through in _getVisibleChild the prior code block will throw an NPE on the line with a.getAccessibleContext();  That NPE will be caught in the second code block and rethrown out to the console.

A null check could be added to the first block but this should not be needed because there is a call to getAccessibleChildrenCount prior to the first block and each of the children should have a valid accessible.

This is not an issue with 8u11.