TYPO3 Core

I have the same error!

1. Translate a page thats a shortcut (doktype = shortcut)
2. For "Shortcut Mode" set "Translation behaviour" to "Custom value".
3. For "Shortcut Target" set "Translation behaviour" to "Custom value".
4. For the translated page the "Shortcut Target" has to be different than in the original page (important)

Expected result:
Exception in Frontend that says:

Attempt to assign property "rootLine" on null

The error occurs because in \TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController $this->tmpl is not set before the function checkTranslatedShortcut gets called!

How I fixed it:
In the class \TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController inside the function checkTranslatedShortcut add the following line so that $this->tmpl is not null anymore:

if (!$this->tmpl instanceof TemplateService) {
   $this->tmpl = GeneralUtility::makeInstance(TemplateService::class, $this->context, null, $this);
}

So the function checkTranslatedShortcut looks like this:

    /**
     * Checks whether a translated shortcut page has a different shortcut
     * target than the original language page.
     * If that is the case, things get corrected to follow that alternative
     * shortcut
     * @param int $languageId
     * @param ServerRequestInterface $request
     */
    protected function checkTranslatedShortcut(int $languageId, ServerRequestInterface $request)
    {
        if (!$this->tmpl instanceof TemplateService) {
            $this->tmpl = GeneralUtility::makeInstance(TemplateService::class, $this->context, null, $this);
        }

        if (!is_null($this->originalShortcutPage)) {
            $originalShortcutPageOverlay = $this->sys_page->getPageOverlay($this->originalShortcutPage['uid'], $languageId);
            if (!empty($originalShortcutPageOverlay['shortcut']) && $originalShortcutPageOverlay['shortcut'] != $this->id) {
                // the translation of the original shortcut page has a different shortcut target!
                // set the correct page and id
                $shortcut = $this->sys_page->resolveShortcutPage($originalShortcutPageOverlay, true);
                $this->id = ($this->contentPid = $shortcut['uid']);
                $this->page = $this->sys_page->getPage($this->id);
                // Fix various effects on things like menus f.e.
                $this->fetch_the_id($request);
                $this->tmpl->rootLine = array_reverse($this->rootLine);
            }
        }
    }

Same here. Your fix works also for me. Thank you!

TYPO3 11.5.30
PHP 8.2