I have been able to get past the above issue. It was caused by me overriding the "nodes" array using my own node type.
So in my custom graph subclassing PointGraph I had this:
public MyNodeType nodes;
Then to make things worse I added the new keyword I think to hide the warning about "hiding" the original variable and things really got weird.
public new MyNodeType nodes;
Now AstarPath thought nodes was null but I could see the nodes array had nodes in it. Even in the debugger it would show incorrectly.
Now I'm just keeping the original array and casting to my type as needed.