fix: correct issue where removing by index failed due to incorrect input validation

Closes #6657

plugins/buildpacks/functions.go

@@ -44,7 +44,7 @@ func rewriteBuildpacksFile(sourceWorkDir string) error {
 
 func validBuildpackURL(buildpack string) (string, error) {
 	if buildpack == "" {
-		return buildpack, errors.New("Must specify a buildpack to add")
+		return buildpack, errors.New("Must specify a buildpack url or reference")
 	}
 
 	reHerokuValue := regexp.MustCompile(`(?m)^([\w-]+\/[\w-]+)$`)

plugins/buildpacks/subcommands.go

@@ -65,7 +65,7 @@ func CommandRemove(appName string, buildpack string, index int) (err error) {
 	}
 
 	buildpack, err = validBuildpackURL(buildpack)
-	if err != nil {
+	if index == 0 && err != nil {
 		return err
 	}