Add check for when can't find parent ID and write to severe error log

lib/services/entry_manager_service.dart

@@ -283,6 +283,11 @@ class EntryManagerService extends ChangeNotifier {
           final parentParentPostId = _postNodes.containsKey(item.parentId)
               ? item.parentId
               : _parentPostIds[item.parentId];
+          if (_postNodes[parentParentPostId] == null) {
+            _logger.severe(
+                'Error finding parent ${item.parentId} for entry ${item.id}');
+            continue;
+          }
           final parentPostNode = _postNodes[parentParentPostId]!;
           postNodesToReturn.add(parentPostNode);
           _parentPostIds[item.id] = parentPostNode.id;