When importing a VariableContent widget with OpenType font, which fails to read the post table of the font file, a NullReferenceException is thrown when the RecalculateContent() method is called.

Workaround: Change the widget font:

foreach (Widget widget in textBoxField.Widgets)
{
    widget.TextProperties.Font = FontsRepository.Helvetica;
    widget.RecalculateContent();
}