This small fix prevents that the forum frontend breaks whenever

custom_less is NULL or unavailable in the database. We cannot rely
on this value to exist or is incorrectly set to null and thus
completely bricking the app.

src/Forum/ForumServiceProvider.php

@@ -94,7 +94,7 @@ public function register()
             $assets->css(function (SourceCollector $sources) {
                 $sources->addFile(__DIR__.'/../../less/forum.less');
                 $sources->addString(function () {
-                    return $this->app->make(SettingsRepositoryInterface::class)->get('custom_less');
+                    return $this->app->make(SettingsRepositoryInterface::class)->get('custom_less') ?? '';
                 });
             });