Fixes a crash when a timeline or thread is backgrounded and brought back with an uninitialized notification or status list.

app/src/main/java/com/keylesspalace/tusky/fragment/NotificationsFragment.java

@@ -71,7 +71,6 @@ public class NotificationsFragment extends SFragment implements
     }
 
     private SwipeRefreshLayout swipeRefreshLayout;
-
     private LinearLayoutManager layoutManager;
     private RecyclerView recyclerView;
     private EndlessOnScrollListener scrollListener;
@@ -134,6 +133,7 @@ public class NotificationsFragment extends SFragment implements
         LocalBroadcastManager.getInstance(context.getApplicationContext())
                 .registerReceiver(timelineReceiver, TimelineReceiver.getFilter(null));
 
+        notifications.clear();
         topLoading = false;
         topFetches = 0;
         bottomLoading = false;

app/src/main/java/com/keylesspalace/tusky/fragment/TimelineFragment.java

@@ -159,6 +159,7 @@ public class TimelineFragment extends SFragment implements
         LocalBroadcastManager.getInstance(context.getApplicationContext())
                 .registerReceiver(timelineReceiver, TimelineReceiver.getFilter(kind));
 
+        statuses.clear();
         topLoading = false;
         topFetches = 0;
         bottomLoading = false;

app/src/main/java/com/keylesspalace/tusky/fragment/ViewThreadFragment.java

@@ -106,6 +106,7 @@ public class ViewThreadFragment extends SFragment implements
         adapter.setMediaPreviewEnabled(mediaPreviewEnabled);
         recyclerView.setAdapter(adapter);
 
+        statuses.clear();
         thisThreadsStatusId = null;
 
         timelineReceiver = new TimelineReceiver(this, this);