Bug reproduces on Master (commit 33cd9986fd tested)
Bug reproduces on 8.0.1 release
Bug does not reproduce on 7.3.1, likely due to lack of implementation on shadergraph reloading on subgraph changes.

Repro steps:
1. Open attached project
2. Open softlock.shadergraph and softlock.subgraph
3. in softlock.shadergraph, drag out the edge from the subgraph's texture2d output to a sample texture 2D node. Do not save the graph.
4. Switch to softlock.subgraph. Change the texture2D output slot to a vector1 output slot. Save the subgraph.
5. Switch to softlock.shadergraph. When it asks to reload, choose yes. Do not save the graph.
6. Switch back to softlock.subgraph. Change the second vector1 output slot (the one you changed from earlier) back to a texture2d slot. Save the subgraph.
7. Switch back to softlock.shadergraph. When it asks to reload, choose yes. Do not save the graph.
8. Drag the edge out from the subgraph's texture2d output to a sample texure 2D node again.

Expected result: no error is thrown, graph is not softlocked
Actual result: graph softlocks, error thrown

Note: this is worth looking into, as even though these are the only steps I found to make this bug happen, there may be alternate steps that will trigger the same bug.