You can connect nodes with output not applicable to the Master Node in the Sub-graph, but deleting that unapplicable node, creates "NullReferenceException" and overall breaks the Shader Editor

Reproduction steps:

1. Open "1100945" project
2. Connect "Sample Texture 2D" to the "Add" node and "Save Asset"
3. Notice that the Shader is Pink (Broken) and you can't connect Sub-graph output to PBR Master Position
4. Delete "Sample Texture 2D" and "Save Asset"
5. Notice "NullReferenceException"
6. Try to delete the connection between the Sub-graph and the PBR Master
7. Notice "NullReferenceException"
8. Try to connect the Sub-graph and the PBR Master
9. Notice "NullReferenceException"
10. Notice that the Editor is very laggy and the connection doesn't disappear on click and produces more "NullReferenceException"

Expected Result: Can't connect, or an error or pop-up appears when trying to, nodes with output not applicable to the Master Node in the Sub-graph, saying why you can't connect those nodes or displaying some visual feedback which indicates that the nodes can't be connected.
Actual Result: You can connect nodes with output not applicable to the Master Node in the Sub-graph, but deleting that unapplicable node, creates "NullReferenceException" and overall breaks the Shader Editor

Fixed in: Shader Graph v5.13.0

Reproduced with: 2019.2.0a5, 2019.1.0b3, 2018.3.6f1
Did not reproduce on: 2017.4.20f2 (No packages)