Summary

<fluent-text-input>'s spellcheck attribute uses a converter whose toView
is not null-safe, so any reflection of spellcheck (setting spellcheck="false",
or SSR-hydrating an element that carries it) throws
TypeError: Cannot read properties of null (reading 'toString'). In practice
spellcheck is unusable on the component.

Root cause

packages/web-components/src/text-input/text-input.base.ts:

@attr({
  converter: {
    fromView: value => (typeof value === 'string' ? ['true', ''].includes(value.trim().toLowerCase()) : null),
    toView: value => value.toString(),
  },
})
public spellcheck!: boolean;

• spellcheck has no default → the backing field is undefined.
• toView calls .toString() directly → throws on null/undefined.
• fromView returns null for any non-string input, which can feed null
  straight back into the field and into toView.

Steps to reproduce

Minimal standalone repro (loads the component from esm.sh, no build/install):

<fluent-text-input id="a" spellcheck="false">Case A</fluent-text-input>
<script type="module">
  import 'https://esm.sh/@fluentui/web-components@3.0.0-rc.14/dist/esm/text-input/define.js';
  await customElements.whenDefined('fluent-text-input');
  // Case B: enqueues attribute reflection -> converter.toView(null) -> null.toString()
  const b = document.createElement('fluent-text-input');
  document.body.appendChild(b);
  queueMicrotask(() => { b.spellcheck = null; });
</script>

Serve over http (custom elements need a real origin) and open DevTools console.

Versions

• Reproduces on @fluentui/web-components@3.0.0-rc.14.
• The same code is present on main (HEAD) — text-input.base.ts still has the
  identical converter and no default value.

Expected

Setting or reflecting spellcheck (including the default/unset state) does not
throw; <fluent-text-input spellcheck="false"> hydrates and disables spelling
suggestions on the internal <input>.

Actual

TypeError: Cannot read properties of null (reading 'toString')
    at Object.toView (.../@fluentui/web-components/dist/esm/text-input/text-input.base.js:417)
    at .../@microsoft/fast-element/dist/esm/components/attributes.js:151
    at tryRunTask (.../@microsoft/fast-element/dist/esm/observation/update-queue.js:19)

Suggested fix

Make the converter null-safe and give the attribute a boolean default, e.g.:

converter: {
  fromView: value => (typeof value === 'string' ? ['true', ''].includes(value.trim().toLowerCase()) : false),
  toView: value => String(!!value),
},

Any equivalent that avoids calling .toString() on null/undefined works.