errors cummulate when instances of Nette\HtmlStringable

[xpavp03]

Version: 3.2.3

Bug Description

When errors added to a form or a control are instances of Nette\HtmlStringable they cummulate.

Steps To Reproduce

 $form = new UI\Form;
 $form->addError(Nette\Utils\Html::fromHtml('1'));
 $form->addError(Nette\Utils\Html::fromHtml('2'));

 $renderer = $form->getRenderer();
 assert($renderer instanceof \Nette\Forms\Rendering\DefaultFormRenderer);

 print $renderer->render($form);

renders the second error twice:

<form action="" method="post">

<ul class="error">
	<li>
		1
	</li>

	<li>
		1
	
		2
	</li>
</ul>


</form>

Possible Solution

It seems to be the problem of clonning and addHtml() in DefaultFormRenderer/doRenderErrors().