code: public EventBus installDefaultEventBus() {
          synchronized (EventBus.class) {
              if (EventBus.defaultInstance != null) {
                  throw new EventBusException("Default instance already exists." +
                          " It may be only set once before it's used the first time to ensure consistent behavior.");
              }
              EventBus.defaultInstance = build();
              return EventBus.defaultInstance;
          }
      }
docs: Installs the default EventBus returned by {@link EventBus#getDefault()} using this builders' values. Must be
  done only once before the first usage of the default EventBus.
  
  @throws EventBusException if there's already a default EventBus instance in place