MaterialAutoComplete handlers are not added again after re-attaching

[kkrupka]

When unloadHandlers() in MaterialAutoComplete is triggered, all handlers are removed, however the global defined HandlerRegistration are not set to null. Means, that after loadHandlers() in MaterialAutoComplete is triggered again on attaching, no handlers are added, because the HandlerRegistration objects are not null. Consequently, for example no suggestion list is shown anymore, because there is no registered click handler. The problem occurred since the change from version 2.1 to 2.2 when in loadHandler() null checks were integrated likeif (itemBoxClickHandler == null).