我收到以下错误Attempttoinvokevirtualmethod'voidandroid.arch.lifecycle.LiveData.observeonanullobjectreference来self的主要fragment中的以下代码部分mReleasesViewModel=ViewModelProviders.of(this).get(ReleasesViewModel.class);mReleasesViewModel.getUpcomingReleases(filter).observe(this,newObserver>(){@Overridepublicvoido