I can pretty much guarantee you that it is a false positive.
Bitwarden is open-source meaning you can see the source code of what make Bitwarden tick, and can view the PowerShell self-hosted install script here.
If you can’t make sense of all the source (genuinely most of us cannot otherwise you are probably a coder) then the other thing you can do is put some trust into 3rd party auditors such as Insight Risk Consulting and Cure53 which Bitwarden has used in the past.
You can trust, but verify as well (which is extremely important IMO) as Bitwarden is very transparent and all their security reports are provided here.
In reading this Bitdefender community post your best option may be to report the error and false positive to Bitdefender to resolve though.
Regardless I hope this post helps to alleviate any worries.