Resolves flag watch crash. Closes #4621

src/engine/impl/geary-generic-imap-folder.vala

@@ -326,6 +326,9 @@ private class Geary.GenericImapFolder : Geary.EngineFolder {
         Gee.List<Geary.Email>? list_remote = yield list_email_by_id_async(low, int.MAX,
             Email.Field.PROPERTIES, ListFlags.FORCE_UPDATE, flag_watch_cancellable);
         
+        if (list_remote == null)
+            return;
+        
         // Build map of emails that have changed.
         foreach (Geary.Email e in list_remote) {
             if (!local_map.has_key(e.id))

src/engine/impl/geary-send-replay-queue.vala

@@ -148,9 +148,9 @@ private class Geary.SendReplayQueue {
                 } catch (Error err) {
                     error("Unable to schedule operation on remote replay queue: %s", err.message);
                 }
+            } else {
+                op.set_ready(null);
             }
-            
-            op.set_ready(null);
         }
     }