Ensure URL segment field type before using its API, and add docs arou…

…nd subsite and fluent domain compatibility

docs/en/introduction.md

@@ -48,3 +48,14 @@ to speak to your website administrator or hosting provider to facilitate this.
 
 
 You can simulate subsite access without setting up virtual hosts by appending ?SubsiteID=<ID> to the request.
+
+### How do Subsite domains work with Fluent domains?
+
+The Subsites module and Fluent translation module both provide the concept of defining "domains" and let you
+configure the host name for it. This functionality is essentially performing the same duty in both modules.
+
+In the "URL segment" field for CMS pages both Subsites and Fluent will add their context to the value. If you
+have a Subsite domain configured but no Fluent domain, Fluent will respect the existing domain and add its
+locale context to the value. If you have a Subsite domain configured and a Fluent domain configure, Fluent will
+use its own domain host name value, and the Subsite domain value will be lost. For this reason, you will need
+to ensure that you use the same host name in both Subsite and Fluent domain entries.

src/Extensions/SiteTreeSubsites.php

@@ -3,6 +3,7 @@
 namespace SilverStripe\Subsites\Extensions;
 
 use Page;
+use SilverStripe\CMS\Forms\SiteTreeURLSegmentField;
 use SilverStripe\CMS\Model\SiteTree;
 use SilverStripe\Control\Controller;
 use SilverStripe\Control\Director;
@@ -150,6 +151,7 @@ public function updateCMSFields(FieldList $fields)
         // replace readonly link prefix
         $subsite = $this->owner->Subsite();
         $nested_urls_enabled = Config::inst()->get(SiteTree::class, 'nested_urls');
+        /** @var Subsite $subsite */
         if ($subsite && $subsite->exists()) {
             // Use baseurl from domain
             $baseLink = $subsite->absoluteBaseURL();
@@ -163,7 +165,7 @@ public function updateCMSFields(FieldList $fields)
             }
 
             $urlsegment = $fields->dataFieldByName('URLSegment');
-            if ($urlsegment) {
+            if ($urlsegment && $urlsegment instanceof SiteTreeURLSegmentField) {
                 $urlsegment->setURLPrefix($baseLink);
             }
         }