@NotNull
  private static Set<VirtualFile> filterOutNulls(@NotNull IndexableSetContributor contributor,
                                                 @NotNull String methodInfo,
                                                 @NotNull Set<VirtualFile> roots) {
    for (VirtualFile root : roots) {
      if (root == null || !root.isValid()) {
        LOG.error(PluginException.createByClass("Please fix " + contributor.getClass().getName() + "#" + methodInfo + ".\n" +
                                                (root == null ? "The returned set is not expected to contain nulls, but it is " + roots
                                                              : "Invalid file returned: " + root), null, contributor.getClass()));
        return new LinkedHashSet<>(ContainerUtil.filter(roots, virtualFile -> virtualFile != null && virtualFile.isValid()));
      }
    }
    return roots;
  }
@NotNull
  private static Set<VirtualFile> filterOutNulls(@NotNull IndexableSetContributor contributor,
                                                 @NotNull String methodInfo,
                                                 @NotNull Set<VirtualFile> roots)