Fix #1453 again (#2169)

* Fix reincarnation of #1453

* fix

python/dgl/transform.py

@@ -1713,21 +1713,14 @@ def to_block(g, dst_nodes=None, include_dst_in_src=True):
             raise ValueError(
                 'Graph has more than one node type; please specify a dict for dst_nodes.')
         dst_nodes = {g.ntypes[0]: dst_nodes}
-    dst_nodes = {
-        ntype: utils.toindex(nodes, g._idtype_str).tousertensor()
-        for ntype, nodes in dst_nodes.items()}
 
-    # dst_nodes is now a dict
-    dst_nodes_nd = []
-    for ntype in g.ntypes:
-        nodes = dst_nodes.get(ntype, None)
-        if nodes is not None:
-            dst_nodes_nd.append(F.to_dgl_nd(nodes))
-        else:
-            dst_nodes_nd.append(nd.NULL[g._idtype_str])
+    dst_node_ids = [
+        utils.toindex(dst_nodes.get(ntype, []), g._idtype_str).tousertensor()
+        for ntype in g.ntypes]
+    dst_node_ids_nd = [F.to_dgl_nd(nodes) for nodes in dst_node_ids]
 
     new_graph_index, src_nodes_nd, induced_edges_nd = _CAPI_DGLToBlock(
-        g._graph, dst_nodes_nd, include_dst_in_src)
+        g._graph, dst_node_ids_nd, include_dst_in_src)
 
     # The new graph duplicates the original node types to SRC and DST sets.
     new_ntypes = (g.ntypes, g.ntypes)
@@ -1735,7 +1728,6 @@ def to_block(g, dst_nodes=None, include_dst_in_src=True):
     assert new_graph.is_unibipartite  # sanity check
 
     src_node_ids = [F.from_dgl_nd(src) for src in src_nodes_nd]
-    dst_node_ids = [F.from_dgl_nd(dst) for dst in dst_nodes_nd]
     edge_ids = [F.from_dgl_nd(eid) for eid in induced_edges_nd]
 
     node_frames = utils.extract_node_subframes_for_block(g, src_node_ids, dst_node_ids)