Some older plugins are no longer compatible with the latest Android Studio (4.1 right now). Also, they seem to have changed the location of the plugins folder with this new version.
You will have to uninstall the plugin. In my case, I couldn't see the plugin listed in the IDE settings. I had to manually find and delete the file.
Currently, plugins are located in the following directories (make sure to change the android version number in the path if your software version is different; versions below 3 are located differently as detailed here):
Windows (ctrl+L in the explorer then paste it):
Delete the plugins there and the error should be gone.