According to this:
https://roaringapps.com/apps
the program BitDefender AntiVirus for Mac is compatible with High Sierra. However, you need to insure that you have the "applicable" version of it. According to this:
https://roaringapps.com/app/bitdefender-antivirus-2012-for-mac
you need to have version 6.1.0.14.
If you have that version, and it still does not work, then there could be other issues with your system. For example, what OS were you using before? Also, you stated that "the crashing has stopped". What "crashing" issues were you having? If you did not completely resolve those before you upgraded, and especially if you upgraded "in place", ie, just have High Sierra overwrite whatever system you already had, there could still be lingering issues. Does not sound like an ideal situation.
Along with that, two more basic questions:
1. Are you making backups to an external device? That is
so, so critical!
2. Have you ever done any disk cleanup, maintenance, and repairs, from a software perspective? You can actually do quite a lot of disk cleanup on your own, and there are some excellent products available (both free and commercial) that can help you with those tasks.