Reproduces on master (7421e9d5b3)
Reproduces in 8.1.0

Repro steps:
1. Create 3 subgraphs, A B and C
2. Drag B into A and connect it, save
3. Drag C into B and connect it, save
4. Open A and save as over C (alternatively, open A, right click the subgraph node and "convert to subgraph" then save over C)
Expected result: it is disallowed or simply creates an invalid subgraph with no other problems
Actual result: unity closes with no crash log and no option to report crash. Fails to open project afterwards, again with no crash log.