code: @PostConstruct
      public void init() {
  
          jfxComboBox.focusedProperty().addListener((o, oldVal, newVal) -> {
              if (!newVal) {
                  jfxComboBox.validate();
              }
          });
  
          ChangeListener<? super Boolean> comboBoxFocus = (o, oldVal, newVal) -> {
              if (!newVal) {
                  jfxEditableComboBox.validate();
              }
          };
          jfxEditableComboBox.focusedProperty().addListener(comboBoxFocus);
          jfxEditableComboBox.getEditor().focusedProperty().addListener(comboBoxFocus);
          jfxEditableComboBox.setConverter(new StringConverter<Label>() {
              @Override
              public String toString(Label object) {
                  return object==null? "" : object.getText();
              }
              @Override
              public Label fromString(String string) {
                  return string == null || string.isEmpty() ? null : new Label(string);
              }
          });
      }
docs: init fxml when loaded.