Make grabs GC, fix a grab related runtime

The following runtime has occured 104 time(s).
runtime error: Cannot execute null.s click().
proc name: Click (/obj/screen/grab/Click)
  source file: screen_objects.dm,73
  usr: Reece Eliza (/mob/living/carbon/human)
  src: the reinforce grab (/obj/screen/grab)