Add extension to correctly support element preview

_config/extensions.yml

@@ -64,3 +64,12 @@ SilverStripe\Subsites\Admin\SubsiteAdmin:
 SilverStripe\SiteConfig\SiteConfigLeftAndMain:
   extensions:
     - SilverStripe\Subsites\Extensions\SubsiteMenuExtension
+
+---
+Name: subsite-preview-elemental
+Only:
+  classexists: DNADesign\Elemental\Models\BaseElement
+---
+DNADesign\Elemental\Models\BaseElement:
+  extensions:
+    - SilverStripe\Subsites\Extensions\BaseElementSubsites

src/Extensions/BaseElementSubsites.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace SilverStripe\Subsites\Extensions;
+
+use SilverStripe\Control\HTTP;
+use SilverStripe\ORM\DataExtension;
+
+/**
+ * Extension for the BaseElement object to add subsites support for CMS previews
+ */
+class BaseElementSubsites extends DataExtension
+{
+    /**
+     * Set SubsiteID to avoid errors when a page doesn't exist on the CMS domain.
+     *
+     * @param string &$link
+     * @param string|null $action
+     * @return string
+     */
+    public function updatePreviewLink(&$link)
+    {
+        // Get subsite ID from the element or from its page. Defaults to 0 automatically.
+        $subsiteID = $this->owner->SubsiteID;
+        if (is_null($subsiteID)) {
+            $page = $this->owner->getPage();
+            if ($page) {
+                $subsiteID = $page->SubsiteID;
+            }
+        }
+
+        $link = HTTP::setGetVar('SubsiteID', intval($subsiteID), $link);
+    }
+}