tidy up asynchronous callbacks associated with gui windows that have been closed

Some tests generate non-fatal NPEs because there are callback events queued on the awt stack that haven't executed. E.g. :
Jalview2XMLTests:
deregistering discoverer listener
INFO - ALL CLOSED
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at jalview.gui.AppJmolBinding$1.run(AppJmolBinding.java:112)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)

or

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at jalview.gui.AppJmolBinding$1.run(AppJmolBinding.java:112)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)

or
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at jalview.gui.AlignmentPanel.getStructureSelectionManager(AlignmentPanel.java:1780)
at jalview.gui.AppJmol.<init>(AppJmol.java:115)
at jalview.gui.StructureViewer.createView(StructureViewer.java:497)
at jalview.project.Jalview2XML$12.run(Jalview2XML.java:7329)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)

similarly for callbacks from jalview.ws2.gui.WebServicesMenuManager.