Runtime fix:  I have no UI, and I must close it.

runtime error: Cannot execute null.close().
proc name: Topic (/obj/item/device/pda/Topic)
  source file: PDA.dm,485
  usr: Garry Myers (/mob/living/carbon/human)
  src: PDA-Garry Myers (Station Engin... (/obj/item/device/pda/engineering)
  call stack:
PDA-Garry Myers (Station Engin... (/obj/item/device/pda/engineering): Topic("src=\[0x2003e95];choice=Messag...", /list (/list))
NeroHarry (/client): Topic("src=\[0x2003e95];choice=Messag...", /list (/list), PDA-Garry Myers (Station Engin... (/obj/item/device/pda/engineering))
NeroHarry (/client): Topic("src=\[0x2003e95];choice=Messag...", /list (/list), PDA-Garry Myers (Station Engin... (/obj/item/device/pda/engineering))
Exploit fix
crash fix