Magento – Fatal error: Method Magento\Ui\TemplateEngine\Xhtml\Result::__toString() must not throw an exception

The error you're getting is indeed triggered in vendor\magento\module-ui\Component\Wrapper\UiComponent.php.

However, it is not triggered at line 0 but when the result is being converted to a string in the following method in the return statement:

protected function _toHtml()
{
    foreach ($this->getChildNames() as $childName) {
        $childBlock = $this->getLayout()->getBlock($childName);
        if ($childBlock) {
            $wrapper = $this->blockWrapperFactory->create([
                'block' => $childBlock,
                'data' => [
                    'name' => 'block_' . $childName
                ]
            ]);
            $this->component->addComponent('block_' . $childName, $wrapper);
        }
    }

    $result = $this->component->render();
    return (string)$result;
}

Here's what you can try to debug your problem:

• check what $result contains before the string conversion and return statement is called
• get informations about the component that is causing an issue by calling $component->getName(), $component->getComponentName() and $component->getData() to help you finding out what the problem is

The problem is the order of the parameters in your parent::__construct() call.
the class your are extending does not have a constructor so it the parent of the parent constructor will be called.
And this parent of parent is \Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection.

It's constructor looks like this:

public function __construct(
    \Magento\Framework\Data\Collection\EntityFactoryInterface $entityFactory,
    \Psr\Log\LoggerInterface $logger,
    \Magento\Framework\Data\Collection\Db\FetchStrategyInterface $fetchStrategy,
    \Magento\Framework\Event\ManagerInterface $eventManager,
    \Magento\Framework\DB\Adapter\AdapterInterface $connection = null,
    \Magento\Framework\Model\ResourceModel\Db\AbstractDb $resource = null
) {
    $this->_eventManager = $eventManager;
    parent::__construct($entityFactory, $logger, $fetchStrategy, $connection);
    $this->_construct();
    $this->_resource = $resource;
    $this->setConnection($this->getResource()->getConnection());
    $this->_initSelect();
}

so the 5th parameter must be something that implements \Magento\Framework\DB\Adapter\AdapterInterface.
In your case, the 5th parameter is something that implementes \Magento\Store\Model\StoreManagerInterface.

Change your parent::__construct() call to this:

    parent::__construct(
        $entityFactory,
        $logger,
        $fetchStrategy,
        $eventManager,
        $connection,
        $resource
    );

the parameters $storeManager and $metadataPool are not part of the classes you extend. If you need them in your own class you can add them in the __construct method like this:

$this->storeManager = $storeManager;
$this->metadataPool = $metadataPool;

and declare them as protected members as you did for $aggregations.