Use number instead of constant, since the constant might not be available yet

extension.json

 {
 	"name": "Semantic Structured Discussions",
 	"type": "semantic",
-	"version": "1.0.0",
+	"version": "1.0.1",
 	"author": [
 		"Marijn van Wezel ([https://wikibase-solutions.com Wikibase Solutions])"
 	],

src/Hooks.php

@@ -47,7 +47,7 @@ final class Hooks {
 		// Enable semantic annotations for pages in the "Topic" namespace
 		// phpcs:ignore MediaWiki.NamingConventions.ValidGlobalName.allowedPrefix
 		global $smwgNamespacesWithSemanticLinks;
-		$smwgNamespacesWithSemanticLinks[NS_TOPIC] = true;
+		$smwgNamespacesWithSemanticLinks[2600] = true;
 	}
 
 	/**