bug free, but bugs out when follow is called with "Cannot invoke this object (REPR: Null; VMNull)" if I change self.id to self!id in method follow in role Follow line 52

ootut2.pm6

our %world = {} ;

role Identifier {
        has $.id is rw ;
}

role Container {
        has @.contents is rw ;
        method add($item) {
                @!contents.push($item) ;
        }
        method remove($item) {
                @!contents = @!contents.grep(!(* ~~ $item)) ;
        }
}


role HasLocation {
        has $.location is rw ;
}

class Room does Identifier does Container {
}


role Movable does HasLocation {
        method move($target) {
                my $source = self.location ;
                my $source-object = %world{$source} ;
                my $target-object = %world{$target} ;
                if ($source-object ~~ Container && $target-object ~~ Container) {
                        my $id = self.id ;
                        $source-object.remove($id) ;
                        self.location = $target ;
                        $target-object.add($id) ;
                        return True ;
                } else {
                        return False ;
                }
        }
}


class Item does Identifier does Movable {
}

role Follow {
        has @.followers is rw ;
        has @.following is rw ;
        method follow($id) {
                my $target-object = %world{$id} ;
                if ($target-object ~~ Follow) && !($id ~~ self.id) {
                        if !($id ~~ any @!following) { @!following.push($id) ; }
                        if !($id ~~ any $target-object.followers) { $target-object.followers.push($id) ; }
                        return True ;
                } else {
                        return False ;
                }

        }
        method unfollow($id) {
        }

}

class Person is Item does Follow {
}


sub populate($object) {
        %world{$object.id} = $object ;
}

our $room1 = Room.new(id => 1, contents => [10,100]) ;
our $room2 = Room.new(id => 2, contents => [200]) ;
our $item10 = Item.new(id => 10, location => 1) ;
our $person100 = Person.new(id => 100, location => 1) ;
our $person200 = Person.new(id => 200, location => 2) ;


populate($room1) ;
populate($room2) ;
populate($item10) ;
populate($person100) ;
populate($person200) ;