You can very much have null errors in F#. Just try this simple snippet:

https://gist.github.com/WimJongeneel/f82b0c4392bd68db3a11d1148c841d95

(Don't get me wrong: I absolutly love F#. But it is still a strict language on top of the CLR and also in essense still mainly OOP with ML style syntax and features sticked on top of it)