Sub-category display not shown in Gutenberg if parent is hidden.

[foonly]

When allowing only a sub-catergory, without the parent category, the sidebar category selector in Gutenberg doesn't show the sub-category. This is not directly a bug in the plugin, as the sub-category is returned in the API call, but Gutenberg doesn't show it, because the category selector appears to only list root categories.

I'm uncertain how this can be fixed within the scope of the plugin, but it would perhaps be preferable to also list the parent categories, so that the sub-categories can be selected. Or if possible, the parent of the sub-category could be changed to 0 in the API call, to cause Gutenberg to display it.