fix: follow reviewer's advice

Author: zhoutongxi

src/server/pegasus_server_impl.cpp

@@ -2750,7 +2750,11 @@ void pegasus_server_impl::update_app_envs_before_open_db(
     update_rocksdb_iteration_threshold(envs);
     update_validate_partition_hash(envs);
     update_user_specified_compaction(envs);
-    if(_db){
+
+    // When backup cluster creates an app via duplication, if source app has env parameters about manual compact,
+    // incomplete replicas may start a manual compact. However, these replicas haven't opened rocksdb successfully yet,
+    // so we need to check if _db exists before starting manual compact to avoid coredump.
+    if (_db != nullptr) {
         _manual_compact_svc.start_manual_compact_if_needed(envs);
     }
     set_rocksdb_options_before_creating(envs);