Check for a program update, the latest release version is 4.6.623.
The file is safe in the Chest, it can do no harm from there and it is best to leave it there for a week or so to ensure that there are no harmful effects having sent it there if it was detected incorrectly. That way you can investigate the problem as required and delete it later.
But to see what is in the Chest right click the avast icon and select Program Settings... then click on the Chest icon. There may be some differences as you are using a dated version of avast.