fix regression from eb1a6a1b6

mouse.lua

@@ -82,7 +82,9 @@ function module.highlight()
 end
 
 function module.hide()
-  w.visible = false
+  if w then
+    w.visible = false
+  end
 end
 
 return module

screen.lua

@@ -89,7 +89,7 @@ end
 local function select_screen(scr_index,move,old_screen)
   if scr_index ~= old_screen then
     local c = last_clients[scr_index]
-    if not pcall(c) then
+    if pcall(c) then
       last_clients[scr_index] = nil
       c = nil
     end