Fix PHPStan issue

src/Dom/LinkManager.php

@@ -3,6 +3,7 @@
 namespace AmpProject\Dom;
 
 use AmpProject\Attribute;
+use AmpProject\Exception\FailedToCreateLink;
 use AmpProject\RequestDestination;
 use AmpProject\Tag;
 use DOMNode;
@@ -258,13 +259,16 @@ public function add($rel, $href, $attributes = [])
             $this->referenceNode = $this->document->viewport;
         }
 
-        /** @var Element $link */
         if ($this->referenceNode) {
             $link = $this->document->head->insertBefore($link, $this->referenceNode->nextSibling);
         } else {
             $link = $this->document->head->appendChild($link);
         }
 
+        if (! $link instanceof Element) {
+            throw FailedToCreateLink::forLink($link);
+        }
+
         $this->links[$this->getKey($link)] = $link;
 
         $this->referenceNode = $link;

src/Exception/FailedToCreateLink.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace AmpProject\Exception;
+
+use RuntimeException;
+
+/**
+ * Exception thrown when a link could not be created.
+ *
+ * @package ampproject/amp-toolbox
+ */
+final class FailedToCreateLink extends RuntimeException implements AmpException
+{
+
+    /**
+     * Instantiate a FailedToCreateLink exception for a link that could not be created.
+     *
+     * @param mixed $link Link that was not as expected.
+     * @return self
+     */
+    public static function forLink($link)
+    {
+        $type = is_object($link) ? get_class($link) : gettype($link);
+        $message = "Failed to create a link via the link manager. "
+                 . "Expected to produce an 'AmpProject\\Dom\\Element', got '$type' instead.";
+
+        return new self($message);
+    }
+}