Can't change vote multiple times on off-chain proposals

[EightRice]

After voting once on an off-chain proposal, attempting to change the vote a second time results in an error. This happens in both production and the deploy preview, so it's not related to PR #35.

Steps to Reproduce

1. Create or find an off-chain proposal with XTZ-weighted voting
2. Vote on a choice
3. Change your vote to a different choice - this works
4. Try to change your vote again
5. Error is thrown

[Screenshot: Error message in console]

Error Details

Error message:

Error: Cannot read properties of null (reading 'walletAddresses')

Backend logs:

[choices.update:tz:error] {
  reqId: 'mi2xgcrv-kiidwm',
  error: "Cannot read properties of null (reading 'walletAddresses')",
  stack: "TypeError: Cannot read properties of null (reading 'walletAddresses')\n" +
    '    at updateChoiceById (C:\\code\\homebase-lite-backend\\components\\choices\\index.js:265:45)\n' +
    '    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)'
}

Root Cause

In components/choices/index.js around line 262-265:

if (isVoted.length > 0) {
  const oldVoteObj = isVoted[0].walletAddresses.find(x => x.address === address);
  oldVote = await ChoiceModel.findById(oldVoteObj.choiceId);

  const oldSignaturePayload = oldVote.walletAddresses[0].payloadBytes  // <-- oldVote is null!

The code tries to find the old vote's choice document, but findById returns null when the choice document doesn't exist. This happens when there's a database inconsistency where the vote reference exists but the actual choice document was deleted or is missing.

Impact

• Users can only change their vote once
• After the first change, they're stuck with that choice
• This affects user experience on longer voting periods where opinions might change

Notes

• This is a pre-existing bug, not introduced by PR feat:Include Staked Balance #35
• The issue appears to be related to orphaned choice references in the database
• Needs null check before accessing oldVote.walletAddresses

Suggested Fix

Add a null check:

if (isVoted.length > 0) {
  const oldVoteObj = isVoted[0].walletAddresses.find(x => x.address === address);
  oldVote = await ChoiceModel.findById(oldVoteObj.choiceId);

  if (!oldVote) {
    throw new Error('Previous vote choice not found - database inconsistency');
  }

  const oldSignaturePayload = oldVote.walletAddresses[0].payloadBytes
  // ... rest of the logic
}

Though this might just surface the underlying database inconsistency issue. May need to investigate why choice documents are getting deleted or not created properly.