Note: fix will need backport to 9.x.x
Note: ways to trigger this bug are:
right clicking
middle mouse clicking
pressing escape

Repro steps:
1. create a new Universal Unlit shader
2. click and drag one of the stack nodes (Position, Normal or Tangent for instance)
3. While dragging, right click, middle mouse click, or press escape

Expected result: it drops the stack node back where it was
Actual result: the stack enters a visibly incorrect state. Undoing throws an error and softlocks the graph, losing any progress since the last save.