Spyder fails to launch if completions are disabled

[mrclary]

When disabling "Enable code completion and linting in the editor", Spyder will not launch.

Steps to Reproduce

1. launch Spyder
2. Uncheck "Enable code completions and linting in the editor" in Spyder Preferences
3. Close Spyder
4. Relaunch Spyder

(spy-dev) >> python bootstrap.py 
Executing Spyder from source checkout
*. Patched sys.path with /Users/rclary/Documents/Python/spyder
*. Patched sys.path with /Users/rclary/Documents/Python/spyder/external-deps/spyder-kernels
*. Patched sys.path with /Users/rclary/Documents/Python/spyder/external-deps/qdarkstyle
*. Patched sys.path with /Users/rclary/Documents/Python/spyder/external-deps/python-language-server
*. Installing PyLS locally
*. Declaring completion providers
*. PyQt5 is detected, selecting
*. Imported Spyder 5.1.0.dev0 - Revision 6648e8632, Branch: 5.x
    [Python 3.9.1 64bits, Qt 5.9.7, PyQt5 5.9.2 on Darwin]
*. Running Spyder
Bootstrap completed in 00:00:01.7434
No QCoreApplication instance found. Application patches not applied. You have to call load_stylesheet function after instantiation of QApplication to take effect. 
Pruned plugin: outline_explorer
Traceback (most recent call last):
  File "/Users/rclary/Documents/Python/spyder/spyder/app/mainwindow.py", line 2139, in create_window
    main.setup()
  File "/Users/rclary/Documents/Python/spyder/spyder/app/mainwindow.py", line 836, in setup
    plugin_deps = solve_plugin_dependencies(enabled_plugins.values())
  File "/Users/rclary/Documents/Python/spyder/spyder/app/solver.py", line 210, in solve_plugin_dependencies
    plugin_deps = [plugin_names[name] for name in deps]
  File "/Users/rclary/Documents/Python/spyder/spyder/app/solver.py", line 210, in <listcomp>
    plugin_deps = [plugin_names[name] for name in deps]
KeyError: 'outline_explorer'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/rclary/Documents/Python/spyder/bootstrap.py", line 278, in <module>
    start.main()
  File "/Users/rclary/Documents/Python/spyder/spyder/app/start.py", line 213, in main
    mainwindow.main(options, args)
  File "/Users/rclary/Documents/Python/spyder/spyder/app/mainwindow.py", line 2286, in main
    mainwindow = create_window(app, splash, options, args)
  File "/Users/rclary/Documents/Python/spyder/spyder/app/mainwindow.py", line 2141, in create_window
    if main.console is not None:
  File "/Users/rclary/Documents/Python/spyder/spyder/app/mainwindow.py", line 1159, in __getattr__
    return self.get_plugin(self._INTERNAL_PLUGINS_MAPPING[attr])
  File "/Users/rclary/Documents/Python/spyder/spyder/app/mainwindow.py", line 168, in get_plugin
    raise SpyderAPIError('Plugin "{}" not found!'.format(plugin_name))
spyder.api.exceptions.SpyderAPIError: Plugin "internal_console" not found!

[ccordoba12]

@mrclary, please note that after fixing this it won't be possible to restore completions unless you reset our settings. We plan to fix that in 5.1.0, to be released (probably) at the end of May.