Note: doesn't reproduce in 7.0.1
Repro steps:
1. Use project with universal RP
2. create a PBR shadergraph, keep it open, with the shadergraph window docked
3. Create a sub graph
4. Drag the sub graph into the shadergraph
5. Delete the sub graph file
6. Plug the sub graph node into a PBR master node input
7. Plug the sub graph node into a different PBR master node input
Expected result: the first time you plug the invalid subgraph into an input, it should plug in successfully and indicate that the graph is broken
Actual result: plugging the node in a second time permanently replaces your left click with the "Create new node" function. From this point on the only part of the Unity UI that is usable is the file - edit - assets - etc. menu at the top. You can save your scene but if you have any other assets that need to be saved, such as shadergraphs or VFX graphs, you will be unable to switch to them, and forced to close unity to get control back. Upon re-opening unity, the shadergraph containing the deleted subgraph will be corrupted.