### 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);
	ItemFont *item = memnew(ItemFont);
	item->def_font = p_def_font;
	item->def_size = true;
	_add_item(item, true);
}
void RichTextLabel::push_font(const Ref<Font> &p_font, int p_size) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ERR_FAIL_COND(p_font.is_null());
	ItemFont *item = memnew(ItemFont);
	item->font = p_font;
	item->font_size = p_size;
	_add_item(item, true);
}
void RichTextLabel::push_normal() {
	ERR_FAIL_COND(theme_cache.normal_font.is_null());
	_push_def_font(NORMAL_FONT);
}
void RichTextLabel::push_bold() {
	ERR_FAIL_COND(theme_cache.bold_font.is_null());
	ItemFont *item_font = _find_font(current);
	_push_def_font((item_font && item_font->def_font == ITALICS_FONT) ? BOLD_ITALICS_FONT : BOLD_FONT);
}
void RichTextLabel::push_bold_italics() {