Steps to reproduced:
1. Create a new project with VFX graph included
2. Create a graph
3. Convert system to subgraph and add additional Start event to it
4. Convert the subgraph with the start event to subgraph overwriting the existing subgraph

Expected result: User should not be allowed to overwrite the existing used graph
Actual result: Graph is overwritten resulting in the editor crashing

Reproduced in: 2020.1.0a5, 2019.3.0b5
No repro in: 2019.2.1f1

Packages reproduced in: 7.1.1
Packages not reproduced in: 6.9.1