I also use Avast so I went there and put in your search term "christmas tree" which was showing in your screen capture...but nothing happened.  What exactly did you search for?  I saw capri pants in your image that didn't show up in  my search...did it show you that when you hit download?  if so it was the file you were trying to download that triggered the warning?

 

“For those who believe, no proof is necessary. For those who don't believe, no proof is possible.”

[Stuart Chase]