"Undeclared arguments passed to ViewHelper" Exception

After updating TYPO3, I get a TYPO3Fluid\Fluid\Core\ViewHelper\Exception "Undeclared arguments passed to ViewHelper ... Valid arguments are."

Tip: Use rector to make these (and other) conversions! Functions for TYPO3 are available, see

• https://usetypo3.com/using-rector-with-typo3.html

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This may be due to an extension using functionality that has been dropped. Using only the TYPO3 core, you should not see this error.

In your extension: If you still use the render() method in your ViewHelper class with arguments, you may want to replace this:

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before:

public function render(Mail $mail, $type = 'web', $function = 'createAction')

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after:

public function initializeArguments()
{
    parent::initializeArguments();

    $this->registerArgument('mail', Mail::class, 'Mail', true);
    $this->registerArgument('type', 'string', 'type: web | mail', false, 'web');
    $this->registerArgument('function', 'string', 'function: createAction | senderMail | receiverMail', false, 'createAction');
}

public function render()
{
    $mail = $this->arguments['mail'];
    $type = $this->arguments['type'] ?? 'web';
    // ...  

}

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Additionally,

• if there is no need to use render() (e.g. unless you need to access $this variables), you may want to switch to renderStatic() for performance reasons (see also this other Stack Overflow answer "What is the difference between render() and renderStatic() ..." for clarification)
• inherit from classes in TYPO3Fluid\Fluid\Core\ViewHelper instead of TYPO3\CMS\Fluid\Core\ViewHelper:

// use TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper;
use TYPO3Fluid\Fluid\Core\ViewHelper\AbstractViewHelper;

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Documentation:

• Developing a Custom ViewHelper

Changelogs:

• Breaking: #82414 - CMS ViewHelper base classes removed (>= TYPO3 9.0)
• Deprecation: #81213 - Render method arguments on ViewHelpers deprecated (>= TYPO3 9.0)
• Breaking: #87193 - Deprecated functionality removed (>= TYPO3 10.0)