Dialog tree view: States are erroneously marked as orphaned

How to reproduce:
- Open the BG2 file `SOLAR.DLG` in the "Edit" tab
- Select state 48 and choose "Show in tree" in the context menu

The state is not supposed to be orphaned, as can be verified with these instructions:
- Open the BG2 file `SOLAR.DLG` in the "Edit" tab
- Select state 47 and choose "Show in tree" in the context menu
- Expand state 47 and any of the associated responses