Adding <hr> breaks loading from automerge

[kythyria]

Attempting to modify the schema in src/schema.ts to add support for <hr> looks initially like it will work, but instead attempting to load a document with one in causes automerge-prosemirror to crash during initialization:

Uncaught Error: Match cannot be null
    set currentMatch traversal.ts:619
    newBlock         traversal.ts:650
    traverseSpans    traversal.ts:597
    pmDocFromSpans   traversal.ts:64
    init             index.ts:84
    <anonymous>      main.ts:40

It crashes in a similar way--the top four frames are identical--while-you-edit if two copies of the page are open pointing at the same document.

The actual substantial change to repro is to copy src/schema.ts and modify the bit about horizontal rules to read

    /// A horizontal rule (`<hr>`).
    horizontal_rule: {
      automerge: {
          block: "__ext__horizontal-rule",
          isEmbed: true,
      },
      group: "block",
      parseDOM: [{ tag: "hr" }],
      toDOM() {
        return hrDOM
      },
    } as NodeSpec,

then pass the resulting schemaAdapter to init().

https://github.com/kythyria/automerge-prosemirror-issue-badblocks is a vanilla JS example sufficient to reproduce, npm run dev, open the URL, try inserting a <hr> using the toolbar (could probably paste one too), observe that reloading the page fails, as does opening it in a second tab.