Why am I getting the error: Cannot update a component (`Unknown`) while rendering a different component (`Edit`)?

I recently upgraded to WordPress 5.9, and I'm now getting the following error in a custom block: Cannot update a component (`Unknown`) while rendering a different component (`Edit`). The code is as below:

export default function Edit( props ) {

    const {attributes, setAttributes} = props
    const {customClass, summaryCount} = attributes
    const childBlockCount = useSelect(select = select('core/block-editor')).getBlockCount(props.clientId))

    const ALLOWED_BLOCKS = [
        ...
    ]

    function Placeholder() {
        return div className=event-summary-placeholder/div
    }

    useEffect(() = {
        if (childBlockCount !== summaryCount)
            setAttributes({summaryCount: childBlockCount})
    }, [childBlockCount])

    const blockProps = useBlockProps({})

    const innerBlocksProps = useInnerBlocksProps(blockProps, {
        allowedBlocks: ALLOWED_BLOCKS,
        templateLock: false
    })

    return (
        aside className='event-aside column-side'
            dl {...useBlockProps({ className: `${customClass} type-s` })}
                {summaryCount === 0  Placeholder/}
                {innerBlocksProps.children}
            /dl
        /aside
    )
}

Most solutions I've found so far recommend using useEffect in some way, but I'm not sure how that would apply here. What's causing this error?