util.quake: check if client exists when moving it to another tag

util/quake.lua

@@ -158,7 +158,10 @@ function quake:toggle()
      if self.followtag then self.screen = awful.screen.focused() end
      local current_tag = self.screen.selected_tag
      if current_tag and self.last_tag ~= current_tag and self.visible then
-         self:display():move_to_tag(current_tag)
+         local c=self:display()
+         if c then
+            c:move_to_tag(current_tag)
+        end
      else
          self.visible = not self.visible
          self:display()