The error appears due to a corrupted configuration file and the solution is to delete it as mentioned by Wojciech Frącz.
It can also appear due to dummy MySQL services (Leftovers from server installations not completely removed). Always make sure you're running the latest Notifier version available (that can be downloaded at mysql downloads page) first.
To solve the dummy MySQL services leftovers:
Open a Run command window and type the following:
Take note of any MySQL service that shouldn't be listed (attempted uninstall before).
Open an Admin rights command window and type the command:
sc delete "< Dummy_service_name_here >".
Restart Notifier and see if that corrected the problem.
If a similar error still appears afterwards, check if the stack trace shows the word CreateScheduledTask somewhere, that would mean something is wrong when Notifier tries to add a schedule task at startup to check for updates of any installed MySQL product.
A workaround is to open in notepad the
settings.config file that was re-generated. Inside the file, look for a property called
"AutoCheckForUpdates" and change the default value of
"False" and run Notifier again, as noted in this post.