Suggested fix: Check for

For pre-existing users, the avatar_manager_avatar_type user property may not have been set. Modify the isset user check to include a isset check.

avantar-manager.php, line 1091

if ( isset( $user ) && isset($user->avatar_manager_avatar_type))
$avatar_type = $user->avatar_manager_avatar_type;
else
return $avatar;