Fixed: cannot handle widgets properly

[GrassH]

When a widget is inserted in the blog content, for example: Products, we will receive an "Invalid template file" error.

"version": "2.0.4"
Query:

query blogPost {
  blogPost(id: "13") {
    filtered_content
  }
}

Result:

{
  "data": {
    "blogPost": {
      "filtered_content": "Error filtering template: Invalid template file: 'Magento_CatalogWidget::product/widget/content/grid.phtml' in module: 'Magento_CatalogWidget' block's name: 'product\\productslist_0'"
    }
  }
}

[ihorvansach]

@GrassH , thank you for your contribution. Could you please provide some notes why you added this additional commit
1b29bf8
with
$themeId = $this->scopeConfig->getValue(
'design/theme/theme_id',
\Magento\Store\Model\ScopeInterface::SCOPE_STORE
);
$theme = $this->themeProvider->getThemeById($themeId);
$this->design->setDesignTheme($theme, Area::AREA_FRONTEND);

[GrassH]

@ihorvansach, when there is a custom theme, in the original code, the template file (.phtml) of the widget cannot be rewritten, and the default template will always be used.