Python kernel dies when adding to subgraph after layout() has been called

[astrofitz]

I have found an issue with pygraphviz that occurs when adding a node to a subgraph after the layout() method has been called on the main graph. The kernel dies when attempting to add the node.

This snippet is able to produce the error:
import pygraphviz as pgv
G = pgv.AGraph()
SG = G.add_subgraph()
G.layout()
SG.add_node('A') # causes python kernel to die

Occurs with pgv version 1.9 on Python 3.10.