I am also getting this same alert. This has all the indications of something that went wrong with the most recent version of Avast for Mac or the virus update. I have never had TuneUpMyMac.
Looking at the processing pass, it has something to do with a false detection in the Assistant Services. I am not sure exactly what it is for, but it seems to be a backend process.
In any case, I think that Avast has to find a fix for this and put out another update. In the meantime, the warning will probably still pop up unless you kill the assistant_service, but I don't know what the consequences of that would be.