Fix runtime error

[HexRabbit]

Fix runtime error caused by close_windows(),

Traceback (most recent call last):
  File "uEmu.py", line 111, in activate
    self.action_handler.handle_menu_action(self.action_type)
  File "uEmu.py", line 1743, in handle_menu_action
    [x.handler() for x in self.MENU_ITEMS if x.action == action]
  File "uEmu.py", line 1743, in <listcomp>
    [x.handler() for x in self.MENU_ITEMS if x.action == action]
  File "uEmu.py", line 1909, in emu_reset
    self.close_windows()
  File "uEmu.py", line 2067, in close_windows
    for viewid in self.memoryViews:
RuntimeError: dictionary changed size during iteration

self.memoryViews[viewid] will be deleted by its OnClose() callback, so we needs to get all the keys first instead of using an iterator, also since all the views will be cleaned by OnClose() callback, there's no need to assign None again in close_windows().