P6opaque: no such attribute '$!tappers_lock'

gistfile1.6pl

sub checkoutGit($repo) {
    say "$repo<name>: git checkout";

    my $proc = Proc::Async.new("git", "clone", $repo<source>, $repo<destination>);
    
    my $stdout;
    my $stderr;
    $proc.stdout.tap(-> $str { $stdout = $stdout ~ $str; }); 
    $proc.stderr.tap(-> $str { $stderr = $stderr ~ $str; }); 

    my $promise = $proc.start;
    await $promise;

    if $promise.status != Kept or $promise.result.exitcode != 0 or $promise.result.signal != 0 { 
        die X::AdHoc.new(payload => "$repo<name>: git checkout failed");
    }   

    return True;
}