@NotNull
  public static Set<VirtualFile> getRootsToIndex(@NotNull IndexableSetContributor contributor) {
    Set<VirtualFile> roots = contributor.getAdditionalRootsToIndex();
    return filterOutNulls(contributor, "getAdditionalRootsToIndex()", roots);
  }
@NotNull
  public static Set<VirtualFile> getRootsToIndex(@NotNull IndexableSetContributor contributor)