code: static List<File> listProductExtensionDirectories() {
          List<File> dirs = new ArrayList<File>();
          for (ProductExtensionInfo info : ProductExtension.getProductExtensions()) {
              // there are some fat tests which constructs incorrect ProductExtensionInfo.
              // in order to avoid the test failure, check null object and skip it if it is null.
              String loc = info.getLocation();
              if (loc != null) {
                  File extensionDir = new File(info.getLocation());
                  if (!extensionDir.isAbsolute()) {
                      File parentDir = Utils.getInstallDir().getParentFile();
                      extensionDir = new File(parentDir, info.getLocation());
                  }
                  dirs.add(extensionDir);
              }
          }
          return dirs;
      }
docs: List all of product extension directory.
  It returns empty List object if there is no product extension.