code: private boolean validate(TextInputControl tf) {
          boolean res = tf.getText().trim().length() == 0;
          ObservableList<String> styleClass = tf.getStyleClass();
          if (res) {
              if (!styleClass.contains("error")) {
                  tf.setStyle("-text-area-background: " + errorColor + ";");
                  styleClass.add("error");
              }
          } else {
              // remove all occurrences:
              tf.setStyle("-text-area-background: " + defaultColor + ";");
              styleClass.removeAll(Collections.singleton("error"));
          }
  
          return res;
      }
docs: Validates the input of the given {@code TextField}
  
  @param tf The {@code TextField} to validate
  @return {@code true} if {@code tf} is <b>NOT</b> ok, {@code false} if it <b>IS</b> ok.