The root cause is that the editor functions
(`retrieve_editable_and_selected_points` etc.) use the evaluated object,
while the operator poll which checks for the active layer uses original
data. The geonodes modifier removes all GP layers. The operator runs
anyway because the original data has an active layer, but then crashes
layer when trying to access the layer in evaluated data.
The fix here is to use original data throughout, which is usually the
data we want to look at for edit/sculpt/paint tools. There is a caveat
that the internal functions like `calculate_view_positions` still should
be using evaluated data for some things, like finding intersections with
other visible strokes. This isn't distinguished clearly atm and might
cause further bugs down the line.
Pull Request: https://projects.blender.org/blender/blender/pulls/129109
f10a2f6f7b