Fix TypeError when loading UI with custom palette selected

[Copilot]

When a custom palette is selected and the page reloads, updateSelectedPalette() is called before custom palettes are added to the DOM, causing TypeError: Cannot read properties of null (reading 'querySelector').

Root Cause

During initialization, the sequence is:

1. loadPalettes() builds palette list with base palettes only
2. requestJson() → readState() → updateUI() → updateSelectedPalette()
3. Custom palettes haven't been added yet (requires lastinfo.cpalcount from step 2)

Changes

Added early return when palette element doesn't exist:

function updateSelectedPalette(s) {
  var parent = gId('pallist');
  var selPaletteInput = parent.querySelector(`input[name="palette"][value="${s}"]`);
  if (selPaletteInput) selPaletteInput.checked = true;

  var selElement = parent.querySelector('.selected');
  if (selElement) selElement.classList.remove('selected');

  var selectedPalette = parent.querySelector(`.lstI[data-id="${s}"]`);
  if (!selectedPalette) return; // palette not yet loaded (custom palette on initial load)
  selectedPalette.classList.add('selected');
  // ... rest of function
}

The palette is correctly selected later when populatePalettes() re-runs with custom palettes.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>UI loading is delayed if custom palette is selected upon page load</issue_title>
<issue_description>### What happened?

There is a circular dependency in index.js:
when the UI is loaded and the selected palette is a custom palette, the call to requestJson() throws an error because it calls readState() but the selected palette is not yet known because the palette list can only be filled including custom palettes after the lastinfo is fetched, which happens in requestJson(). So the UI throws this error:

To Reproduce Bug

select a custom palette, reload the UI

Expected Behavior

resolve custom palettes without throwing the error.

Install Method

Binary from WLED.me

What version of WLED?

nightly build

Which microcontroller/board are you seeing the problem on?

ESP32-C3

Relevant log/trace output

TypeError: Cannot read properties of null (reading 'querySelector')
    at updateSelectedPalette ((index):7:63528)
    at updateUI ((index):7:62253)
    at readState ((index):7:68130)
    at (index):7:71556

Anything else?

the UI still load correctly, it just takes longer due to timeouts and there is the error thrown.

Code of Conduct

• I agree to follow this project's Code of Conduct</issue_description>

<agent_instructions>please check if there is a way to avoid this error without adding a lot of code. keep it brief.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes UI loading is delayed if custom palette is selected upon page load #5204

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[coderabbitai]

Important

Review skipped

Bot user detected.

To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[DedeHai]

tested working