TypeError: Argument 1 passed to _mailsystem_html_to_text() must be an instance of DOMNode, null given

Thank you for reporting this and documenting your fix, which is actually in mailsystem, not in htmlmail.

Here's what we're looking at:

/**
 * Transform an HTML string into plain text, preserving the structure of the
 * markup. Useful for preparing the body of a node to be sent by e-mail.
 *
 * The output will be suitable for use as 'format=flowed; delsp=yes' text
 * (RFC 3676) and can be passed directly to drupal_mail() for sending.
 *
 * We deliberately use variable_get('mail_line_endings', MAIL_LINE_ENDINGS)
 * rather than "\r\n".
 *
 * This function provides suitable alternatives for the following tags:
 *
 * <a> <address> <b> <blockquote> <br /> <caption> <cite> <dd> <div> <dl> <dt>
 * <em> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <ol> <p> <pre> <strong>
 * <table> <tbody> <td> <tfoot> <thead> <tr> <u> <ul>
 *
 * The following tag attributes are supported:
 * - <a href=...>: Hyperlink destination urls.
 * - <li value=...>: Ordered list item numbers.
 * - <ol start=...>: Ordered list start number.
 *
 * @param $string
 *   The string to be transformed.
 * @param $allowed_tags
 *   (optional) If supplied, a list of tags that will be transformed. If
 *   omitted, all supported tags are transformed.
 *
 * @return
 *   The transformed string.
 *
 * @see drupal_mail()
 */
function mailsystem_html_to_text($string, $allowed_tags = NULL) {
  $eol = variable_get('mail_line_endings', MAIL_LINE_ENDINGS);
  // Cache list of supported tags.
  static $supported_tags;
  if (!isset($supported_tags)) {
    $supported_tags = array(
      'a', 'address', 'b', 'blockquote', 'br', 'cite', 'dd', 'div', 'dl',
      'dt', 'em', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'hr', 'i', 'li',
      'ol', 'p', 'pre', 'strong', 'table', 'td', 'tr', 'u', 'ul',
    );
  }

  // Make sure only supported tags are kept.
  $allowed_tags = isset($allowed_tags) ? array_intersect($supported_tags, $allowed_tags) : $supported_tags;

  // Parse $string into a DOM tree.
  $dom = filter_dom_load($string);
  $notes = array();
  // Recursively convert the DOM tree into plain text.
  $text = _mailsystem_html_to_text($dom->documentElement, $allowed_tags, $notes);
  // Hard-wrap at 1000 characters (including the line break sequence)
  // and space-stuff special lines.
  $text = mailsystem_wrap_mail($text, array('max' => 1000 - strlen($eol), 'hard' => TRUE));
  // Change non-breaking spaces back to regular spaces, and trim line breaks.
  // chr(160) is the non-breaking space character.
  $text = str_replace(chr(160), ' ', trim($text, $eol));
  // Add footnotes;
  if ($notes) {
    // Add a blank line before the footnote list.
    $text .= $eol;
    foreach ($notes as $url => $note) {
      $text .= $eol . '[' . $note . '] ' . $url;
    }
  }
  return $text;
}

htmlmail does use this function, but it's only passing a string. The function then calls filter_dom_load() to create the contents of the $dom variable.

Can you try to find out what the strings are that are being passed in to mailsystem_html_to_text() either on your test server or your production server?

Maybe you're looking at a situation like
#1494910: mailsystem_html_to_text() function did not return any text or
#1786546: mailsystem_html_to_text() function did not return any text
and this throws an error now with PHP 7 and it used to just return an empty string under earlier versions?

Also, please try the 2.70 version!