How to retrieve and process the data from the open dialog form?

Question

I'm trying to process the response data in the open dialog but got stuck in this the retrieval.
I followed the step by step procedure given in this link: https://developers.google.com/chat/how-tos/dialogs but unfortunately, it didn't work as well.

Here is the sample code:

My goal is to get the data from the dialog form then process it. My pain point is in the code: event.common.formInputs.firstnumber.stringInputs.value[0] where it returns undefined reading value.

function openDialog(event) {
  return {
    "action_response": {
        "type": "DIALOG",
        "dialog_action": {
          "dialog": {
            "body": {
              "sections": [
                {
                  "header": "Addition Calculator:",
                  "widgets": [
                    {
                  "textInput": {
                    "label": "First Number",
                    "type": "SINGLE_LINE",
                    "name": "firstnumber"
                  }
                },
                {
                  "textInput": {
                    "label": "Second Number",
                    "type": "SINGLE_LINE",
                    "name": "secondnumber"
                  }
                },
                {
                  "textInput": {
                    "label": "Third Number",
                    "type": "SINGLE_LINE",
                    "name": "thirdnumber"
                  }
                },
                {
                      "buttonList": {
                        "buttons": [
                          {
                            "text": "Submit",
                            "onClick": {
                              "action": {
                                "function": "giveAnswer"
                              }
                            }
                          }
                        ]
                      },
                      "horizontalAlignment": "END"
                    }
                  ]
                }
              ]
            }
          }
        }
    }
  };
}


function giveAnswer(event) {
  var firstterm = parseFloat(event.common.formInputs.firstnumber.stringInputs.value[0])
  var secondterm = parseFloat(event.common.formInputs.secondnumber.stringInputs.value[0])
  var thirdterm = parseFloat(event.common.formInputs.thirdnumber.stringInputs.value[0])
  var sum = firstterm+secondterm+thirdterm
  return {
      "cardsV2": [{
        "cardId": "addContact",
        "card": {
          "header": {
            "title": "The SUM is:",
            "subtitle": "Answer",
            "imageUrl": "https://images.emojiterra.com/google/noto-emoji/v2.034/128px/1f4f1.png",
            "imageType": "SQUARE"
          },
          "sections": [
            {
              "widgets": [
                {
                  "textParagraph": {
                        "text": sum
                      }
                    }
                  ]
                }
              ]
            }
          }
        ]
      }
    }

I tried the example in here but didn't work as well. https://developers.google.com/chat/how-tos/dialogs#receive_form_data_from_dialogs