Issue #3513456 by grimreaper: String prop type: convert from, default case

Closes #3513456

src/Plugin/UiPatterns/PropType/StringPropType.php

@@ -37,6 +37,7 @@ class StringPropType extends PropTypePluginBase {
       'url' => $value,
       'identifier' => $value,
       'string' => $value,
+      'default' => (string) $value,
     };
   }