What version of RH are you running? Classic or New UI (RH2019)? Are you all patched up? Check Help > Updates to check.
I'm running RH2019 and it's all patched up.
Nobody can help with just that information. What you need to do follow the instructions and then repeat what you were doing when the error occurred. If it then happens again, post back with information about what you are trying to do. Then hopefully someone can help.
No need to be rude; if it happened under just one circumstance I would have included that - but it happens at random with no correlation to a specific action.
The only error I hate more (from a different application) is "Object reference not set to an instance of an object."
As near as I can tell, they both translate to "Did you try switching it off and on again? Yes? Okay, try thumping it a few times".
Have you tried to do a series of actions without saving each time?
I have tried that and also tried the same series of actions while saving after each one. There is no direct correlation between this error and any specific action being performed - it happens at random. Many times, after I have restarted Robohelp (as the error states to do) the error will throw again immediately after the restart.
What's rude? I simply stated that no one could help based on just that information. I then suggested what to do first and if it happened again to come back with some more information so that I or someone else could try to help based on what you told us.
if it happened under just one circumstance I would have included that - but it happens at random with no correlation to a specific action
OK but I had no way of knowing your thinking. I can only go on what was posted.
Some error in the CSS file could contribute. I'm only testing RH2019 New occasionally, but projects converted from old versions of RH seem to have lots of strange items included that are fine/ignored in old versions. But New UI is more strict so weird things can happen. Perhaps run your css files through an external CSS checking tool to identify issues. Be careful about removing all identified issues, in case there's some proprietary RH code that's needed (I haven't done enough digging to know if there is anything, but thought I'd mention it, just in case.)
Amber's suggestion is a good one. If the erroring is frequent enough, go to Reports > Topic List and select all topics (top left check box). Right click and select Properties. There you can apply the default css to all topics. Obviously they will look rubbish but you can try a few things to see if anything can create the error.
The code in your topics will not have been changed so later you can reverse the process but take a copy of your project if in doubt.
I now recall an application error, albeit not as random, which was tracked down to upgraded list styles.