fix registration placeholder replacement

Author: mkazlauskas

paper-input.html

@@ -256,7 +256,10 @@
       var version = Polymer.Element ? 'v1' : 'v0';
       var inputTemplate = Polymer.DomModule.import('paper-input', 'template#' + version);
       var inputPlaceholder = template.content.querySelector('#template-placeholder');
-      inputPlaceholder.parentNode.replaceChild(inputTemplate.content, inputPlaceholder);
+      if (inputPlaceholder) {
+        inputPlaceholder.parentNode.replaceChild(inputTemplate.content, inputPlaceholder);
+      }
+      // else it's already been processed, probably in superclass
     },
 
     /**