Using form fields relies on helvetica font

[srsbiz]

TCPDF/tcpdf.php

Line 9941 in d4adef4

$font = $this->getFontBuffer((($this->pdfa_mode) ? 'pdfa' : '') .'helvetica');

When using custom font defined in constant PDF_FONT_NAME_MAIN, helvetica is not loaded into font buffer and using form fields produces following error:

PHP Warning:  Trying to access array offset on value of type bool in .../vendor/tecnickcom/tcpdf/tcpdf.php on line 9942

I think it should use value of PDF_FONT_NAME_MAIN, but I don't know how exacly it works with pdfa or if helvetica is required somewhere else and should be always loaded?

MRE:

<?php

error_reporting(-1);

require __DIR__ . '/vendor/autoload.php';

define('K_TCPDF_EXTERNAL_CONFIG', true);
define('PDF_FONT_NAME_MAIN', 'freesans');
define('PDF_FONT_NAME_DATA', 'freesans');

$pdf = new TCPDF();
$pdf->AddPage();
$pdf->TextField('test', 40, 10);
$pdf->output(__DIR__ . '/test.pdf', 'F');