CollisionGroupsEditor.rbxm does not use GetService but instead directly indexes the child by its name. This has been deemed as non-canonical.
Plugin.Components.Gui (118):
game.Selection:Set(getPartsInGroup(group.Name))
Plugin.Components.Gui (178)
self.SelectionChangedConn = game.Selection.SelectionChanged:Connect(function()
Plugin.getSelectedParts (4)
for _, object in pairs(game.Selection:Get()) do
These should be changed to game:GetService("Selection").
Along with being non-canonical (the primary issue), this becomes an issue if you change service names at runtime (please don’t turn this into a discussion of whether you should or not).