private @NotNull Set<VirtualFile> getReadOnlyUsagesFiles() {
    Set<Usage> usages = getReadOnlyUsages();
    Set<VirtualFile> result = new HashSet<>();
    for (Usage usage : usages) {
      if (usage instanceof UsageInFile usageInFile) {
        VirtualFile file = usageInFile.getFile();
        if (file != null && file.isValid()) result.add(file);
      }

      if (usage instanceof UsageInFiles usageInFiles) {
        ContainerUtil.addAll(result, usageInFiles.getFiles());
      }
    }
    for (UsageTarget target : myTargets) {
      VirtualFile[] files = target.getFiles();
      if (files == null) continue;
      ContainerUtil.addAll(result, files);
    }
    return result;
  }
private @NotNull Set<VirtualFile> getReadOnlyUsagesFiles()