fix(extui): check if buffers/windows exist before deleting (#34886)

Problem: Disabling vim._extui may try to delete non-existent windows/buffers.
Solution: Check that window/buffer is valid before deleting.

runtime/lua/vim/_extui.lua

@@ -65,11 +65,15 @@ function M.enable(opts)
   if ext.cfg.enable == false then
     -- Detach and cleanup windows, buffers and autocommands.
     for _, win in pairs(ext.wins) do
+      if api.nvim_win_is_valid(win) then
         api.nvim_win_close(win, true)
       end
+    end
     for _, buf in pairs(ext.bufs) do
+      if api.nvim_buf_is_valid(buf) then
         api.nvim_buf_delete(buf, {})
       end
+    end
     api.nvim_clear_autocmds({ group = ext.augroup })
     vim.ui_detach(ext.ns)
     return