'gui_objects have no parent' crash at line 907: initialize(this) cannot be
called from sender==@conlobby@ context (external mail). Fix:
1. Replace initialize(this)+mail(-9998,parent) in MAIL_LOAD_AUTO_MISSION
with mail(MAIL_LOAD_AUTO_MISSION_DONE=-8888, this) -- a self-mail.
2. Add MAIL_LOAD_AUTO_MISSION_DONE handler in sender==this block, which
safely calls initialize(this)+mail(-9998,parent) from the correct context.
Also repaired a corrupt duplicate MAIL_PREVIOUS_MISSION_PARAMS block
that was left orphaned by an earlier edit (the original body of
MAIL_LOAD_AUTO_MISSION had been inserted there when the handler was
moved from sender==this to sender==@conlobby@).
No rebuild required (script-only).