@Nullable
  public Library getLibrary(final String libraryName, @NotNull String libraryLevel) {
/* the null check is added only to prevent NPE when called from getLibrary */
    final LibrariesModifiableModel model = myLevel2Providers.get(libraryLevel);
    return model == null ? null : findLibraryModel(libraryName, model);
  }
@Nullable
  public Library getLibrary(final String libraryName, @NotNull String libraryLevel)