Prevent crash

app/src/main/java/eu/faircode/email/FragmentMessages.java

@@ -70,7 +70,6 @@ import androidx.constraintlayout.widget.Group;
 import androidx.fragment.app.FragmentActivity;
 import androidx.fragment.app.FragmentTransaction;
 import androidx.lifecycle.Lifecycle;
-import androidx.lifecycle.LifecycleOwner;
 import androidx.lifecycle.Observer;
 import androidx.lifecycle.ViewModelProviders;
 import androidx.localbroadcastmanager.content.LocalBroadcastManager;
@@ -452,11 +451,10 @@ public class FragmentMessages extends FragmentEx {
                     FragmentActivity activity = getActivity();
                     if (activity != null) {
                         ViewModelMessages modelMessages = ViewModelProviders.of(activity).get(ViewModelMessages.class);
-                        LifecycleOwner owner = (viewType == AdapterMessage.ViewType.THREAD ? getViewLifecycleOwner() : activity);
                         if (selectionTracker.hasSelection())
-                            modelMessages.removeObservers(viewType, owner);
+                            modelMessages.removeObservers(viewType, getViewLifecycleOwner());
                         else
-                            modelMessages.observe(viewType, owner, observer);
+                            modelMessages.observe(viewType, getViewLifecycleOwner(), observer);
                     }
 
                     if (selectionTracker.hasSelection()) {
@@ -1758,8 +1756,7 @@ public class FragmentMessages extends FragmentEx {
 
         // Sort changed
         final ViewModelMessages modelMessages = ViewModelProviders.of(getActivity()).get(ViewModelMessages.class);
-        LifecycleOwner owner = (viewType == AdapterMessage.ViewType.THREAD ? getViewLifecycleOwner() : getActivity());
-        modelMessages.removeObservers(viewType, owner);
+        modelMessages.removeObservers(viewType, getViewLifecycleOwner());
 
         DB db = DB.getInstance(getContext());
         LivePagedListBuilder<Integer, TupleMessageEx> builder = null;
@@ -1847,8 +1844,8 @@ public class FragmentMessages extends FragmentEx {
 
         builder.setFetchExecutor(executor);
 
-        modelMessages.setMessages(viewType, builder.build());
-        modelMessages.observe(viewType, owner, observer);
+        modelMessages.setMessages(viewType, getActivity(), builder.build());
+        modelMessages.observe(viewType, getViewLifecycleOwner(), observer);
     }
 
     private Observer<PagedList<TupleMessageEx>> observer = new Observer<PagedList<TupleMessageEx>>() {

app/src/main/java/eu/faircode/email/ViewModelMessages.java

@@ -34,23 +34,30 @@ import androidx.paging.PagedList;
 public class ViewModelMessages extends ViewModel {
     private Map<Boolean, LiveData<PagedList<TupleMessageEx>>> messages = new HashMap<>();
 
-    void setMessages(AdapterMessage.ViewType viewType, LiveData<PagedList<TupleMessageEx>> messages) {
-        boolean thread = (viewType == AdapterMessage.ViewType.THREAD);
+    void setMessages(AdapterMessage.ViewType viewType, LifecycleOwner owner, final LiveData<PagedList<TupleMessageEx>> messages) {
+        final boolean thread = (viewType == AdapterMessage.ViewType.THREAD);
         this.messages.put(thread, messages);
+
+        // Keep list up-to-date for previous/next navigation
+        messages.observe(owner, new Observer<PagedList<TupleMessageEx>>() {
+            @Override
+            public void onChanged(PagedList<TupleMessageEx> messages) {
+            }
+        });
+
+        owner.getLifecycle().addObserver(new LifecycleObserver() {
+            @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
+            public void onDestroyed() {
+                Log.i("Removed model thread=" + thread);
+                ViewModelMessages.this.messages.remove(thread);
+            }
+        });
     }
 
     void observe(AdapterMessage.ViewType viewType, LifecycleOwner owner, Observer<PagedList<TupleMessageEx>> observer) {
         if (owner.getLifecycle().getCurrentState().isAtLeast(Lifecycle.State.RESUMED)) {
             final boolean thread = (viewType == AdapterMessage.ViewType.THREAD);
             messages.get(thread).observe(owner, observer);
-
-            owner.getLifecycle().addObserver(new LifecycleObserver() {
-                @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
-                public void onDestroyed() {
-                    Log.i("Removed model thread=" + thread);
-                    messages.remove(thread);
-                }
-            });
         }
     }