Fix: null check in setSmallerLargerStatus to prevent TypeError on wheel scroll

[swapnachoudhary43]

Fixes #6436

Problem

When scrolling the mouse wheel on the canvas before the UI is
fully initialized, setSmallerLargerStatus crashes with:

TypeError: Cannot read properties of null (reading 'children')
    at Activity.setSmallerLargerStatus (activity.js:2188:43)
    at Activity._doSmallerBlocks (activity.js:2176:24)
    at doSmallerBlocks (activity.js:2145:28)
    at HTMLCanvasElement.__wheelHandler (activity.js:2525:55)

Root Cause

this.smallerContainer and this.largerContainer can be null
when the wheel event fires before containers are initialized.

Fix

Added a single null guard at the top of setSmallerLargerStatus:

if (!this.smallerContainer || !this.largerContainer) return;

Testing

• All 123 test suites passing
• Manually verified no console errors on mouse wheel scroll

Checklist

• I have read the contributing guidelines
• Tests pass locally
• Fix is minimal and focused

[github-actions]

❌ Some Jest tests failed. Please check the logs and fix the issues before merging.

Failed Tests:

synthutils.test.js

[kartikktripathi]

Initially, the code looks good, but while trying to reproduce the bug, I was not able to. Can you give the exact steps to reproduce or maybe a clip of this error showing up while you test it?

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[swapnachoudhary43]

Hi @kartikktripathi! Thanks for the review. Here are the exact steps to reproduce:

Open Music Blocks in browser
Immediately scroll the mouse wheel on the canvas (within the first 1-2 seconds before UI fully loads)
Open DevTools Console (F12)
You will see: TypeError: Cannot read properties of null (reading 'children') at Activity.setSmallerLargerStatus

This happens because smallerContainer and largerContainer are initialized as null in setupDependencies() and only get assigned later in _setupPaletteMenu(). If the wheel event fires in that window, the crash occurs.
The fix adds a single null guard at the top of setSmallerLargerStatus to return early if containers aren't ready yet.

[kartikktripathi]

Hey, I tried to reproduce it, but I'm still not able to do so on the master branch. Can you record a video of this bug if possible, and attach it here? That'll be helpful.

[Ashutoshx7]

hy any updates on this