Try this:
- Click the link on the line following File > Preferences > More preferences can be edited directly in the file. This will open the Arduino15 (or similar name depending on OS) folder.
- Delete all files and folders under the Arduino15 folder except for preferences.txt. Please be very careful when deleting things from your computer. When in doubt back up!
- Close all Arduino IDE windows then restart the Arduino IDE
Note that this will remove your ESP8266 boards package. You will need to reinstall it following the instructions they provided.