Error on startup with non-clean config file (from previous files/projects open?) and recent master builds

[CAM-Gerlach]

Issue Report Checklist

• Searched the issues page for similar reports
• Read the relevant sections of the Spyder Troubleshooting Guide and followed its advice
• Reproduced the issue after updating with conda update spyder (or pip, if not using Anaconda)
• Tried basic troubleshooting (if a bug/error)
  • Restarted Spyder
  • Reset preferences with spyder --reset
  • Tried the other applicable steps from the Troubleshooting Guide
• Completed the Problem Description, Steps to Reproduce and Version sections below

Problem Description

In recent dev builds of master (past few days) I've been getting the following error on launch, that otherwise doesn't seem to have ill effect. However, I can't easily reproduce it in a clean config (--safe-mode) so I'm not sure if its a bug or something with my configuration switching back and forth between Spyders. This was with just one horizontally split Editor pane. I started getting it before I ever undocked anything or used "New Window"after I'd launched Spyder with your #8192 once and then switched back to master and launched another Spyder, and now I get it every time I launch master or the #8192 branch without --safe-mode.

@jnsebgosselin reported he gets this as well, even without any split.

  File "C:\Users\C. A. M. Gerlach\Documents\dev\SpyderDev\spyder\spyder\plugins\editor\plugin.py", line 1312, in toogle
    conf_name)
  File "C:\Users\C. A. M. Gerlach\Documents\dev\SpyderDev\spyder\spyder\plugins\editor\plugin.py", line 1329, in _toggle_checkable_action
    if self.editorstacks:
AttributeError: 'Editor' object has no attribute 'editorstacks'

Versions

• Spyder version: master
• Python version: 3.6.6
• Qt version: 5.9,6
• PyQt version: 5.9.2
• Operating System name/version: Windows 8.1 x64

[ccordoba12]

Fixed by #8249.