Cannot read properties of null reading classlist ошибка

I am getting an error

TypeError: Cannot read property 'classList' of null

I am not really sure what am I doing wrong in manipulating DOM element for each form field. My form field has an stated name and ID of form and each input field has a name and id of the field as well. But I am still getting an error.

What am i doing wrong?

My script:

document.querySelector("#form").addEventListener("submit", function(e){
//create variable for contact form url
var formURL = 'melonForm.php';
//prevent default submission
event.preventDefault();
//define form fields
var melonForm = {
    'firstName'              : document.querySelector('input[name=firstName]').value,
    'lastName'             : document.querySelector('input[name=lastName]').value,
    'companyName'       : document.querySelector('input[name=companyName]').value,
    'companyAddress'       : document.querySelector('input[name=companyAddress]').value,
    'city'       : document.querySelector('input[name=city]').value,
    'state'       : document.querySelector('select[name=state]').value,
    'zipcode'       : document.querySelector('input[name=zipcode]').value,
    'emailAddress'       : document.querySelector('input[name=emailAddress]').value,
    'phoneNumber'       : document.querySelector('input[name=phoneNumber]').value,
}

//define request variable
var formRequest = new Request(formURL, {
    method: 'POST', 
    body: melonForm, 
    headers: new Headers()
});

//fetch
fetch(formRequest)
.then(function(formResponse) {
    return formResponse.json();
  })
.then(function(data) {
    //handle server responses
    if ( ! data.success) {
    //handle error messages
        //handle error message for firstName
        console.log(data);
        if (data.errors.firstName) {
            document.getElementById("firstName").classList.add("has-error");
            document.getElementById("firstName").appendChild('<div class="help-block">' + data.errors.firstName + '</div>');
        }
        //handle errors for lastName
        if (data.errors.lastName) {
            document.getElementById("lastName").classList.add("has-error");
            document.getElementById("lastName").appendChild('<div class="help-block">' + data.errors.lastName + '</div>');
        }
        //handle errors for companyName
        if (data.errors.companyName) {
            document.getElementById("companyName").classList.add("has-error");
            document.getElementById("companyName").appendChild('<div class="help-block">' + data.errors.companyName + '</div>');
        }
        //handle errors for companyAddress
        if (data.errors.companyAddress) {
            document.getElementById("companyAddress").classList.add("has-error");
            document.getElementById("companyAddress").appendChild('<div class="help-block">' + data.errors.companyAddress + '</div>');
        }
        //handle errors for city
        if (data.errors.city) {
            document.getElementById("city").classList.add("has-error");
            document.getElementById("city").appendChild('<div class="help-block">' + data.errors.city + '</div>');
        }
        //handle errors for state
        if (data.errors.state) {
            document.getElementById("state").classList.add("has-error");
            document.getElementById("statea").appendChild('<div class="help-block">' + data.errors.state + '</div>');
        }
        //handle errors for zipcode
        if (data.errors.zipcode) {
            document.getElementById("zipcode").classList.add("has-error");
            document.getElementById("zipcode").appendChild('<div class="help-block">' + data.errors.zipcode + '</div>');
        }
        //handle errors for emailAddress
        if (data.errors.emailAddress) {
            document.getElementById("emailAddress").classList.add("has-error");
            document.getElementById("emailAddress").appendChild('<div class="help-block">' + data.errors.emailAddress + '</div>');
        }
        //handle errors for phoneNumber
        if (data.errors.phoneNumber) {
            document.getElementById("phoneNumber").classList.add("has-error");
            document.getElementById("phoneNumber").appendChild('<div class="help-block">' + data.errors.phoneNumber + '</div>');
        }
        // handle errors for captcha ---------------
        if (data.errors.captcha) {
            swal({
                title: "Error!",
                text: data.errors.captcha,
                icon: "error",
            });
        }
        // handle errors for phpmailer ---------------
        if (data.message) {
            swal({
                title: "Error!",
                text: data.message,
                icon: "error",
                });
        }   
    else {
    //handle success messages
        swal({
            title: "Success!",
            text: data.message,
            icon: "success",
            });
    document.getElementById("form").reset();    
    }
}
});
})

I know that the field firstName does exist in the html but not sure why the javascript is not able to read the element.

ADDED HTML:

<html>
<head>
    <title>Melon Form</title>
    <link rel="stylesheet" href="https://netdna.bootstrapcdn.com/bootstrap/3.0.3/css/bootstrap.min.css"> <!-- load bootstrap via CDN -->

    <script src="https://unpkg.com/sweetalert/dist/sweetalert.min.js"></script>

</head>
<body>
<div class="col-sm-6 col-sm-offset-3">

    <h1>Contact Form</h1>

    <!-- OUR FORM -->
    <form name="form" id="form" action="melonForm.php" method="POST">

        <!-- FIRST NAME -->
        <div id="firstName-group" class="form-group">
            <label for="firstName">First Name:</label>
            <input type="text" class="form-control" name="firstName" placeholder="Henry Pym">
            <!-- errors will go here -->
        </div>

        <!-- LAST NAME -->
        <div id="lastName-group" class="form-group">
            <label for="lastName">Last Name:</label>
            <input type="text" class="form-control" name="lastName" placeholder="Henry Pym">
            <!-- errors will go here -->
        </div>

        <!-- COMPANY NAME -->
        <div id="companyName-group" class="form-group">
            <label for="companyName">Company Name:</label>
            <input type="text" class="form-control" name="companyName" placeholder="Henry Pym">
            <!-- errors will go here -->
        </div>

        <!-- COMPANY ADDRESS -->
        <div id="companyAddress-group" class="form-group">
            <label for="companyAddress">Company Address:</label>
            <input type="text" class="form-control" name="companyAddress" placeholder="Henry Pym">
            <!-- errors will go here -->
        </div>

        <!-- CITY -->
        <div id="city-group" class="form-group">
            <label for="city">City:</label>
            <input type="text" class="form-control" name="city" id="city" placeholder="Henry Pym">
            <!-- errors will go here -->
        </div>

        <div id="state-group" class="form-group">
        <label for="state">State</label>
        <select id="state" name="state" class="form-control">
            <option value="" selected>Choose...</option>
            <option value="AL">Alabama</option>
            <option value="AK">Alaska</option>
            <option value="AZ">Arizona</option>
            <option value="AR">Arkansas</option>
            <option value="CA">California</option>
            <option value="CO">Colorado</option>
            <option value="CT">Connecticut</option>
            <option value="DE">Delaware</option>
            <option value="DC">District Of Columbia</option>
            <option value="FL">Florida</option>
            <option value="GA">Georgia</option>
            <option value="HI">Hawaii</option>
            <option value="ID">Idaho</option>
            <option value="IL">Illinois</option>
            <option value="IN">Indiana</option>
            <option value="IA">Iowa</option>
            <option value="KS">Kansas</option>
            <option value="KY">Kentucky</option>
            <option value="LA">Louisiana</option>
            <option value="ME">Maine</option>
            <option value="MD">Maryland</option>
            <option value="MA">Massachusetts</option>
            <option value="MI">Michigan</option>
            <option value="MN">Minnesota</option>
            <option value="MS">Mississippi</option>
            <option value="MO">Missouri</option>
            <option value="MT">Montana</option>
            <option value="NE">Nebraska</option>
            <option value="NV">Nevada</option>
            <option value="NH">New Hampshire</option>
            <option value="NJ">New Jersey</option>
            <option value="NM">New Mexico</option>
            <option value="NY">New York</option>
            <option value="NC">North Carolina</option>
            <option value="ND">North Dakota</option>
            <option value="OH">Ohio</option>
            <option value="OK">Oklahoma</option>
            <option value="OR">Oregon</option>
            <option value="PA">Pennsylvania</option>
            <option value="RI">Rhode Island</option>
            <option value="SC">South Carolina</option>
            <option value="SD">South Dakota</option>
            <option value="TN">Tennessee</option>
            <option value="TX">Texas</option>
            <option value="UT">Utah</option>
            <option value="VT">Vermont</option>
            <option value="VA">Virginia</option>
            <option value="WA">Washington</option>
            <option value="WV">West Virginia</option>
            <option value="WI">Wisconsin</option>
            <option value="WY">Wyoming</option>
        </select>
        </div>

        <!-- ZIPCODE -->
        <div id="zipcode-group" class="form-group">
            <label for="zipcode">Zipcode:</label>
            <input type="text" class="form-control" name="zipcode" id="zipcode" placeholder="12345">
            <!-- errors will go here -->
        </div>  

        <!-- EMAIL ADDRESS -->
        <div id="emailAddress-group" class="form-group">
            <label for="emailAddress">Email Address:</label>
            <input type="text" class="form-control" name="emailAddress" placeholder="rudd@avengers.com">
            <!-- errors will go here -->
        </div>

        <!-- PHONE NUMBER -->
        <div id="phoneNumber-group" class="form-group">
            <label for="phoneNumber">Phone Number:</label>
            <input type="text" class="form-control" name="phoneNumber" id="phoneNumber" placeholder="12345">
            <!-- errors will go here -->
        </div>  

        <!-- MESSAGE -->
        <div id="message-group" class="form-group">
            <label for="message">Message:</label>
            <input type="text" class="form-control" name="message" placeholder="Ant Man">
            <!-- errors will go here -->
        </div>

        <!-- GOOGLE RECAPTCHA -->
        <div class="form-group">
            <div class="g-recaptcha" data-sitekey="SECRET_KEY"></div>
        </div>

        <button type="submit" class="btn btn-success">Submit <span class="fa fa-arrow-right"></span></button>

    </form>

</div>
<script src="melonForm.js" defer></script> <!-- load our javascript file -->
</body>
<script src='https://www.google.com/recaptcha/api.js'></script>
</html>