I'm not aware of Nabble having been hacked. Maybe you personally have been hacked? I don't think I've ever seen such a Google Chrome message, and not surprising since I don't use Google Chrome. :-)
Be careful. It could be to mislead you and steal your password.
Next time it happens make a screenshot and show it to someone with computer skills before you do something, if you don't know what to do.
(Press the print screen button on your keyboard, then start "Microsoft Paint", then click "paste", then save the file).
Good side point Re browser. I hear Google is bad because it runs so many processes.
Which browser is safest for ambient RF with less CPU action happening? (not sure that is the right term)
These days if you don't pay for an antivirus pack,
there are sometimes features missing that are needed (like a firewall for example).
So using a free anti-virus is an increased risk.
Not all anti-virus software is the same either.
I tested 39, and could find none without security risks,
even "the best" have flaws. Not sure why, it doesn't seem hard to fix certain problems.
It almost seems deliberate.
It's also about updates.
Using an old Windows that can't be updated or not updating your windows is of coarse no good,
not regularly updating your browser and all software that connects to the internet,
and not updating your modem/router firmware, is all risky.
But as they say: the weakest link remains the person using the computer.