The current java.awt.Component event dispatching is not weak thread safe.
But it is desired by the JavaBean concept. And many parts of the awt design,
epecailly the AWTEventMulticaster, reflect the thread safe feature. See
the following code for the problem,

    protected void processFocusEvent(FocusEvent e) {
        if (focusListener != null) {
            /* TIMING WINDOW */
            int id = e.getID();
            switch(id) {
              case FocusEvent.FOCUS_GAINED:
                focusListener.focusGained(e);
                break;
              case FocusEvent.FOCUS_LOST:
                focusListener.focusLost(e);
                break;
            }
        }
    }

The focusListener may get nulled because of the removeFocusListener is called from a second thread, which will generate a NullPointerException for this case.

The fix is trivial. We just need to use a local variable to keep the reference.

    protected void processFocusEvent(FocusEvent e) {
        FocusListener listener = focusListener;
        if (listener != null) {
            int id = e.getID();
            switch(id) {
              case FocusEvent.FOCUS_GAINED:
                listener.focusGained(e);
                break;
              case FocusEvent.FOCUS_LOST:
                listener.focusLost(e);
                break;
            }
        }
    }