Fix an error when deleting an account

2018-07-17 15:41:55 21814 140135444791360 ERROR    nxdrive.manager    Could not start the engine 65181d94493911e8879a0800279b8e3e
Traceback (most recent call last):
  File "manager.py", line 299, in start
    engine.start()
  File "engine/engine.py", line 587, in start
    raise FsMarkerException()
nxdrive.engine.engine.FsMarkerException
2018-07-17 15:41:55 21814 140135444791360 ERROR    nxdrive.commandline Error executing 'launch'
Traceback (most recent call last):
  File "commandline.py", line 491, in handle
    return handler(options)
  File "commandline.py", line 526, in launch
    app = self._get_application(console=console)
  File "commandline.py", line 512, in _get_application
    return Application(self.manager)
  File "gui/application.py", line 83, in __init__
    self.init_checks()
  File "gui/application.py", line 428, in init_checks
    self.manager.start()
  File "manager.py", line 293, in start
    for uid, engine in self._engines.items():
RuntimeError: dictionary changed size during iteration