Topoff's fail, it's just a matter of when. I've lost two tanks to a topoff failure in the past, and so I design them differently now.
Never give them an unlimited supply of water.
I give the topoff no more than 25% of the tank volume, so if it fails at most it can only spill so much, and only adjust the salinity so much.
Never have just one sensor.
I have a sensor for normal water level, this is the most expensive and reliable one. I then have a low level, and high level sensor. If the low level sensor trips it shuts down the return pump and alerts me by text. If the high level trips it shuts down the topoff until manually reset, and alerts me by text.
Different sensors fail in different ways.
Optical sensors tend to fail on because something drys on the lens part, but almost never fail off. Great for water too high applications.
Mechanical/electrical sensors tend to jam, never use these in a Kalk Tank. Kalk deposits will jam these.
The best sensors I've found are the Avast Pressure based sensors. They have been very reliable for me.