It sounds like you still have some confusion about references in your code. If I were you I would examine that code very carefully, and step through it with a debugger to make sure there aren’t any references to goview1 or gosubgraph1 when the drop happens in goview2/gosubgraph2.
Do you override GoObject.OnSelectionDropped in your subgraph class in order to implement the addition of dropped objects into your subgraph?
You might also want to look at the Planogrammer sample, for an example of having drag-and-drop add or move things between "containers".
It's entirely possible that this problem is related to your problem with drag-and-drop.