public void registerPageLanguage(final Project project, final XmlFile containingFile, final String languageName) {
    WriteCommandAction.writeCommandAction(project).withName(getFamilyName()).run(() -> {
      final PsiFileFactory factory = PsiFileFactory.getInstance(project);
      final XmlFile dummyFile = (XmlFile)factory.createFileFromText("_Dummy_.fxml", XmlFileType.INSTANCE,
                                                                    "<?language " + languageName + "?>");
      final XmlDocument document = dummyFile.getDocument();
      if (document != null) {
        final XmlProlog prolog = document.getProlog();
        final Collection<XmlProcessingInstruction> instructions = PsiTreeUtil.findChildrenOfType(prolog, XmlProcessingInstruction.class);
        LOG.assertTrue(instructions.size() == 1);
        final XmlDocument xmlDocument = containingFile.getDocument();
        if (xmlDocument != null) {
          final XmlProlog xmlProlog = xmlDocument.getProlog();
          if (xmlProlog != null) {
            final PsiElement element = xmlProlog.addBefore(instructions.iterator().next(), xmlProlog.getFirstChild());
            xmlProlog.addAfter(PsiParserFacade.getInstance(project).createWhiteSpaceFromText("\n\n"), element);
          }
        }
      }
    });
  }
public void registerPageLanguage(final Project project, final XmlFile containingFile, final String languageName)