livebook branced section bug

gistfile1.txt

# Bug Repo Case

## Tools

### How to use this notebook

This notebook will crash when the index of the first branch is set to a 0 index

```
<!-- livebook:{"branch_parent_index":0} -->
```

You can work around it by manually chainging the index to point to itself

```
<!-- livebook:{"branch_parent_index":1} -->
```

<!-- livebook:{"branch_parent_index":0} -->

## Setup a bogus rover

```elixir
IO.puts("This wont print with index 0")
```