Throw ElementError when the menu of the header is missing but a toggle is present

[romaricpascal]

We need to keep the early return if the toggle is absent, as it's not mandatory for the header to have a navigation.

Closes #4130

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-4.6.0.min.css	118.07 KiB
dist/govuk-frontend-4.6.0.min.js	47.1 KiB
dist/govuk-frontend-ie8-4.6.0.min.css	79.27 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	76.96 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	72.29 KiB
packages/govuk-frontend/dist/govuk/all.mjs	3.84 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	37.41 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.83 KiB

Modules

File	Size
all.mjs	68.39 KiB
components/accordion/accordion.mjs	21.9 KiB
components/button/button.mjs	4.71 KiB
components/character-count/character-count.mjs	21.64 KiB
components/checkboxes/checkboxes.mjs	5.69 KiB
components/error-summary/error-summary.mjs	6.01 KiB
components/exit-this-page/exit-this-page.mjs	16.6 KiB
components/header/header.mjs	3.83 KiB
components/notification-banner/notification-banner.mjs	4.54 KiB
components/radios/radios.mjs	4.68 KiB
components/skip-link/skip-link.mjs	3.73 KiB
components/tabs/tabs.mjs	9.07 KiB

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Action run for 827ba90

[colinrotherham]

Thanks for doing this

Helps the compiler out a lot (narrows the types down) when adding to class fields after the checks

Strict mode will no longer warn that they might be null etc

[colinrotherham]

Approved, but can you fix my daft previous suggestion? Sorry