Fixes Z-level fuckery

code/modules/modular_computers/file_system/programs/medical/suit_sensors.dm

@@ -42,6 +42,10 @@
 	for(var/z in (data["map_levels"] | T.z))  // Always show crew from the current Z even if we can't show a map
 		data["crewmembers"] += crew_repository.health_data(z)
 
+	if(!data["map_levels"].len)
+		to_chat(user, "<span class='warning'>The crew monitor doesn't seem like it'll work here.</span>")
+		return
+
 	ui = SSnanoui.try_update_ui(user, src, ui_key, ui, data, force_open)
 	if(!ui)
 		ui = new(user, src, ui_key, "crew_monitor.tmpl", "Crew Monitoring Computer", 900, 800, state = state)