### 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 
_validate_line_caches();
	int para_count = 0;
	int to_line = main->first_invalid_line.load();
	for (int i = 0; i < to_line; i++) {
		if (main->lines[i].char_offset < p_char && p_char <= main->lines[i].char_offset + main->lines[i].char_count) {
			return para_count;
		} else {
			para_count++;
		}
	}
	return -1;
}
int RichTextLabel::get_total_character_count() const {
	// 
	int tc = 0;
	Item *it = main;
	while (it) {
		if (it->type == ITEM_TEXT) {
			ItemText *t = static_cast<ItemText *>(it);
			tc += t->text.length();
		} else if (it->type == ITEM_NEWLINE) {
			tc++;
		} else if (it->type == ITEM_IMAGE) {
			tc++;
		}
		it = _get_next_item(it, true);
	}
	return tc;
}
int RichTextLabel::get_total_glyph_count() const {