Uncaught TypeError: Cannot read property 'value' of null: beginSendMessage  @   contact.js:14

function beginSendMessage() {
    if (BlogEngine.$('<%=txtAttachment.ClientID %>') && BlogEngine.$('<%=txtAttachment.ClientID %>').value.length > 0)
        return true;

    if (!Page_ClientValidate('contact'))
        return false;

    var recaptchaResponseField = document.getElementById('recaptcha_response_field');
    var recaptchaResponse = recaptchaResponseField ? recaptchaResponseField.value : "";

    var recaptchaChallengeField = document.getElementById('recaptcha_challenge_field');
    var recaptchaChallenge = recaptchaChallengeField ? recaptchaChallengeField.value : "";

    var name = BlogEngine.$('<%=txtName.ClientID %>').value;
    var email = BlogEngine.$('<%=txtEmail.ClientID %>').value;
    var subject = BlogEngine.$('<%=txtSubject.ClientID %>').value;
    var message = BlogEngine.$('<%=txtMessage.ClientID %>').value;
    var sep = '-||-';
    var arg = name + sep + email + sep + subject + sep + message + sep + recaptchaResponse + sep + recaptchaChallenge;
    WebForm_DoCallback('__Page', arg, endSendMessage, 'contact', onSendError, false)

    BlogEngine.$('<%=btnSend.ClientID %>').disabled = true;

    return false;
}