Fatal error: Unsupported operand types

Yes, this is from calling theme('table_cell') from a Twig file

In theme.inc I did the following:

  if (isset($variables) && !is_array($variables)) {
    print_r($hook);
    var_export($variables);
  }

Which produced:

table_cell

Drupal\Core\Template\TwigReference::__set_state(array(
   'data' => '<span class="element-invisible">Enabled</span>',
   'class' => 
  array (
    0 => 'checkbox',
  ),
   'attributes' => 
  array (
    'class' => 
    array (
      0 => 'checkbox',
    ),
  ),
   'header' => true,
))

So the $variables array is actually a TwigReference object. So I think this is related to #1842160: Consider appropriate usage of theme() from within Twig templates. I'll do some more investigating, but let me try to solve that other issue and perhaps this one will also be resolved.