fix: allow overwriting of the Title field via customised data

docs/en/01_Title.md

@@ -4,13 +4,32 @@ are vital properties in order to be placed well in a google search.
 
 With silverstripe cms, you typically let the framework render the title tag
 automatically (which corresponds to the page name) or overwrite the default
-title in the `Page.ss` template. While both strategies work, we have found that,
-especially with page objects, the title tag is often different from the page name.
+title in the `Page.ss` template.
 
-We therefore add an additional field to the page object, the `meta title`, which
-lets an editor overwrite the title specifically. This is the default setup when
-installing this module, but you can change this behaviour by using the following
-configuration options:
+We have found that, especially with page objects, the title tag is often
+different from the page name. Therefore, this module adds an additional field to
+the page object, the `meta title`, which lets an editor overwrite the title
+specifically.
+
+While you can let Silverstripe handle the rendering of the title tag using the
+`$MetaTags` in the head section, this may become unwieldy when combined with
+controllers that try to overwrite the Title, as this Title is then not picked
+up by the module. We therefore recommend to render the title in your `Page.ss`
+template as follows:
+
+```html
+    $MetaTags(false)
+    <title>
+      <% if $SEOSource %>
+        $SEOSource.SEOTitle
+      <% else %>
+        $Title | $SiteConfig.Title
+      <% end_if %>
+    </title>
+```
+This will only render a specific title when a source has been set (in the DOAP
+case) or when the current controller action is `index` (in the normal case of
+rendering a page). Otherwise, the title will fall back to the normal behaviour.
 
 ## Disable the metatitle field
 

src/Extensions/SEOSiteTreeExtension.php

@@ -3,6 +3,7 @@
 
 use SilverStripe\ORM\DataObject;
 use SilverStripe\ORM\DataExtension;
+use SilverStripe\Control\Controller;
 use SilverStripe\CMS\Model\SiteTree;
 use Syntro\SEO\Extensions\SEOExtension;
 
@@ -28,11 +29,12 @@ public function getSEOSource()
     {
         if ($this->source) {
             return $this->source;
-        } else {
+        } else if (Controller::curr()->getAction() == 'index') {
             $owner = $this->getOwner();
             $this->setSEOSource($owner);
             return $owner;
         }
+        return null;
     }
 
     /**
@@ -57,7 +59,7 @@ public function MetaComponents(&$tags)
         /** @var SiteTree $owner */
         $owner = $this->getOwner();
         $source = $this->getSEOSource();
-        if (!$source->hasExtension(SEOExtension::class)) {
+        if (!$source || !$source->hasExtension(SEOExtension::class)) {
             return $tags;
         }
         // Add a title