fix rendering bug in view sources

Closes #3465262

modules/ui_patterns_views/src/Plugin/UiPatterns/Source/ViewsSourceBase.php

@@ -111,12 +111,14 @@ abstract class ViewsSourceBase extends SourcePluginBase {
     if (empty($output)) {
       return ['#markup' => ''];
     }
-    // Value needs to be render array so slots can handle multiple sources.
+    // We use #children to avoir filtering.
     if ($output instanceof MarkupInterface) {
-      return ['#markup' => $output->__toString()];
+      return [
+        '#children' => (string) $output,
+      ];
     }
     if (is_scalar($output)) {
-      return ['#markup' => $output];
+      return ['#children' => $output];
     }
     return $output;
   }