ERROR: cleanup: There are still MemoryPool allocs in use at exit!

I have 2 classes in the GDNative plugin (Vertex and ListDigraph) and ListDigraph has a method `add_vertex` that return a Ref

```
Ref ListDigraph::add_vertex() {
return Ref::__internal_constructor(Vertex::_new());
}
```

And is bound to the GDScript using:

```
register_method("add_vertex", &ListDigraph::add_vertex);
```

Having this code gives some silent errors:
1) At editor start-up:
```
ERROR: set_path: Another resource is loaded from path: res://bin/list_digraph.gdns (possible cyclic resource inclusion)
At: core/resource.cpp:79.
```

2) At editor shut down
```
WARNING: cleanup: ObjectDB Instances still exist!
At: core/object.cpp:2092.
ERROR: cleanup: There are still MemoryPool allocs in use at exit!
At: core/pool_vector.cpp:70.
```

What am I doing wrong here?