Crash when hiding an 'Instance Position' or 'Pointer Position' node feeding a value

Set up this simple graph:

Select the 'Instance Position' node (or instead, wire it up to use the 'Pointer Position' node and select that) then hide it with Ctrl+H (Cmd+H on Mac). This results in a crash.

There are similar cases where this does *not* crash (these exhibit the behavior we'd expect from the above cases):
- A 'Vec2 Value' node instead
- An 'Instance Index' node feeding into the Rotation input of the Transform node

Reproduce the crash using the described graph and Ctrl+H or Cmd+H on the selected Instance Position or Pointer Position node. Compare the result with the Vec2 Value and Instance Index cases; done means hiding either position node no longer crashes and behaves consistently with those cases.

Written by the indexing model from the issue text.