### 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_FRAME);
	ItemFrame *cell = static_cast<ItemFrame *>(current);
	ERR_FAIL_COND(!cell->cell);
	cell->border = p_color;
}
void RichTextLabel::set_cell_size_override(const Size2 &p_min_size, const Size2 &p_max_size) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type != ITEM_FRAME);
	ItemFrame *cell = static_cast<ItemFrame *>(current);
	ERR_FAIL_COND(!cell->cell);
	cell->min_size_over = p_min_size;
	cell->max_size_over = p_max_size;
}
void RichTextLabel::set_cell_padding(const Rect2 &p_padding) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type != ITEM_FRAME);
	ItemFrame *cell = static_cast<ItemFrame *>(current);
	ERR_FAIL_COND(!cell->cell);
	cell->padding = p_padding;
}
void RichTextLabel::push_cell() {