Steps to reproduce the issue

Create a contact.
Create a single Contact menu item to display this contact.

Modify the Contact Publishing parameters.
For example:
Set it to Unpublished
Or, set it to Published and modify publish up or down so as to not display the contact when the menu item is clicked to therefore obtain a 404.

Expected result

404 Contact not found

Actual result

Argument 1 passed to Joomla\Registry\Registry::merge() must be an instance of Joomla\Registry\Registry, null given, called in /Applications/MAMP/htdocs/newfolder/joomla40/components/com_contact/src/Model/ContactModel.php on line 132

Trying to solve the issue

Modifying ContactModel.php line 132 to use registry
From

$temp->merge($contact->params);
$contact->params = $temp;

to

$registry = new Registry($contact->params);
$temp->merge($registry);
$contact->params = $temp;

We now get
Argument 1 passed to Joomla\Registry\Registry::merge() must be an instance of Joomla\Registry\Registry, null given, called in /Applications/MAMP/htdocs/newfolder/joomla40/components/com_contact/src/View/Contact/HtmlView.php on line 147

Modifying Contact/HtmlView.php

Adding use Joomla\Registry\Registry;
Then change line 147 from

$temp->merge($item->params);
$item->params = $temp;

to

$registry = new Registry($item->params);
$temp->merge($registry);
$item->params = $temp;

Result:

500 Exception: Contact not found in /Applications/MAMP/htdocs/newfolder/joomla40/components/com_contact/src/Model/ContactModel.php:249 Stack trace: #0 /Applications/MAMP/htdocs/newfolder/joomla40/libraries/src/MVC/View/AbstractView.php(146): Joomla\Component\Contact\Site\Model\ContactModel->getItem() #1 /Applications/MAMP/htdocs/newfolder/joomla40/components/com_contact/src/View/Contact/HtmlView.php(118): Joomla\CMS\MVC\View\AbstractView->get('Item') #2 /Applications/MAMP/htdocs/newfolder/joomla40/libraries/src/MVC/Controller/BaseController.php(691): Joomla\Component\Contact\Site\View\Contact\HtmlView->display() #3 /Applications/MAMP/htdocs/newfolder/joomla40/components/com_contact/src/Controller/DisplayController.php(81): Joomla\CMS\MVC\Controller\BaseController->display(false, Array) #4 /Applications/MAMP/htdocs/newfolder/joomla40/libraries/src/MVC/Controller/BaseController.php(729): Joomla\Component\Contact\Site\Controller\DisplayController->display() #5 /Applications/MAMP/htdocs/newfolder/joomla40/libraries/src/Dispatcher/ComponentDispatcher.php(146): Joomla\CMS\MVC\Controller\BaseController->execute('display') #6 /Applications/MAMP/htdocs/newfolder/joomla40/components/com_contact/src/Dispatcher/Dispatcher.php(45): Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch() #7 /Applications/MAMP/htdocs/newfolder/joomla40/libraries/src/Component/ComponentHelper.php(389): Joomla\Component\Contact\Site\Dispatcher\Dispatcher->dispatch() #8 /Applications/MAMP/htdocs/newfolder/joomla40/libraries/src/Application/SiteApplication.php(206): Joomla\CMS\Component\ComponentHelper::renderComponent('com_contact') #9 /Applications/MAMP/htdocs/newfolder/joomla40/libraries/src/Application/SiteApplication.php(245): Joomla\CMS\Application\SiteApplication->dispatch() #10 /Applications/MAMP/htdocs/newfolder/joomla40/libraries/src/Application/CMSApplication.php(231): Joomla\CMS\Application\SiteApplication->doExecute() #11 /Applications/MAMP/htdocs/newfolder/joomla40/includes/app.php(63): Joomla\CMS\Application\CMSApplication->execute() #12 /Applications/MAMP/htdocs/newfolder/joomla40/index.php(36): require_once('/Applications/M...') #13 {main} 

I.e. instead of getting a 404 from the ContactModel

if (empty($data))
{
	throw new \Exception(Text::_('COM_CONTACT_ERROR_CONTACT_NOT_FOUND'), 404);
}

we get the result from the HtmlView.php code

		// Check for errors.
		if (count($errors = $this->get('Errors')))
		{
			throw new GenericDataException(implode("\n", $errors), 500);
		}

Note:

1. There may be other places where Registry should be used
2. The same (or very similar) code for article uses the 404 from the ArticleModel. No registry error.