This indicates something has gone wrong with your ESP8266 core for Arduino installation. To make sure it has been fully uninstalled you need to take some drastic action. Be aware this is somewhat advanced and will remove all boards you have installed via Boards Manager:
- 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!
- (In the Arduino IDE) Tools > Board > Boards Manager
- Wait for the downloads to finish.
- Scroll down until you see the ESP8266 entry. Click on it.
- Install any other boards that you need.
- Click "Install".
- Wait for installation to finish.
- Click "Close"