### Godot version

master ([06ccbfe](https://github.com/godotengine/godot/commit/06ccbfe8eec6e840db3df2360dc2804386f940b8))

### System information

PopOS 22.04

### Issue description

Sometimes the doc page is broken when the engine is closed and then restarted.
Closing the doc and opening it again fixes it.

![2023-05-27_22-15](https://github.com/godotengine/godot/assets/3624853/ed202a55-03e8-420a-bc1c-ccfc7f7cd02a)


### Steps to reproduce

- Open a documentation page for any node
- Restart Godot

### Minimal reproduction project

NA

EDIT : Added the commit hash @Calinou 
_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ERR_FAIL_COND(p_string.is_empty());
	ERR_FAIL_COND(p_font.is_null());
	ERR_FAIL_COND(p_size <= 0);
	ItemDropcap *item = memnew(ItemDropcap);
	item->text = p_string;
	item->font = p_font;
	item->font_size = p_size;
	item->color = p_color;
	item->ol_size = p_ol_size;
	item->ol_color = p_ol_color;
	item->dropcap_margins = p_dropcap_margins;
	_add_item(item, false);
}
void RichTextLabel::_push_def_font_var(DefaultFont p_def_font, const Ref<Font> &p_font, int p_size) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemFont *item = memnew(ItemFont);
	item->def_font = p_def_font;
	item->variation = true;
	item->font = p_font;
	item->font_size = p_size;
	item->def_size = (p_size <= 0);
	_add_item(item, true);
}
void RichTextLabel::_push_def_font(DefaultFont p_def_font) {