Issue #3457112: FormatTextFilter needs to return a string

src/Plugin/TypedDataFilter/FormatTextFilter.php

@@ -60,7 +60,7 @@ class FormatTextFilter extends DataFilterBase {
    * {@inheritdoc}
    */
   public function filter(DataDefinitionInterface $definition, $value, array $arguments, BubbleableMetadata $bubbleable_metadata = NULL) {
-    return check_markup($value, $arguments[0]);
+    return (string) check_markup($value, $arguments[0]);
   }
 
 }