Messages that are replies and do not use formatting cannot be edited more than once

[20kdc]

Describe the bug

Um... See title or reproduction instructions?

Reproduction

1. Markdown formatting is enabled in settings. (Unsure if essential)
2. Reply to a message. The reply must not use markdown features (so it doesn't generate a formatted body, perhaps?)
3. Edit the reply. Do not use markdown features.
4. Edit the reply again. This will cause the error and the edit will silently fail.

Importantly, m.new_content will not have formatted_body in messages that break this way.

Expected behavior

Uh... no error and for the message to be edited.

Platform and versions

1. OS: Ubuntu MATE 22.04.2 LTS
2. Browser: Firefox 102.12.0esr (64-bit)
3. Cinny version: v2.2.6 (app.cinny.in)
4. Matrix homeserver: matrix.org

Additional context

Materials that may or may not be useful:

Example of the source to a message that cannot be further edited:

{
  "content": {
    "body": "> <@20kdc:matrix.org> This is a message.\n\n * This is a reply to that message that I am going to edit.\nThis is an edit.",
    "format": "org.matrix.custom.html",
    "formatted_body": "<mx-reply><blockquote><a href=\"https://matrix.to/#/!yDMeZGveGjjPHJVagK%3Amatrix.org/%24unsiVC0GP7GC_W60VU6UIUtzqWeo11OxkV1xLhsX0_M\">In reply to</a><a href=\"https://matrix.to/#/%4020kdc%3Amatrix.org\">@20kdc:matrix.org</a><br />This is a message.</blockquote></mx-reply> * This is a reply to that message that I am going to edit.<br>This is an edit.",
    "m.new_content": {
      "body": "This is a reply to that message that I am going to edit.\nThis is an edit.",
      "msgtype": "m.text"
    },
    "m.relates_to": {
      "event_id": "$9DyIp-qkkWV_h7WHyYyvMaG-LflZUEynS0y_IUKsQmg",
      "rel_type": "m.replace"
    },
    "msgtype": "m.text"
  },
  "origin_server_ts": 1687725600719,
  "sender": "@20kdc:matrix.org",
  "type": "m.room.message",
  "unsigned": {
    "age": 297,
    "transaction_id": "m1687725600670.2"
  },
  "event_id": "$_ByLPPp9PABr6f7DOoxMf74_d5Wk7m4aoO_N9uWezmw",
  "room_id": "!yDMeZGveGjjPHJVagK:matrix.org"
}

[kfiven]

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'substring')
    at RoomsInput.getContent (RoomsInput.js:236:35)
    at RoomsInput.sendEditedMessage (RoomsInput.js:412:26)
    at onSave (Message.jsx:811:39)
    at onSubmit (Message.jsx:308:76)
    at HTMLUnknownElement.callCallback2 (react-dom.development.js:3945:14)
    at Object.invokeGuardedCallbackDev (react-dom.development.js:3994:16)
    at invokeGuardedCallback (react-dom.development.js:4056:31)
    at invokeGuardedCallbackAndCatchFirstError (react-dom.development.js:4070:25)
    at executeDispatch (react-dom.development.js:8243:3)
    at processDispatchQueueItemsInOrder (react-dom.development.js:8275:7)

[jivanpal]

Confirmed affects me on v2.2.6 Flatpak on Pop OS 22.04.