Backport of PR-8772 for Magento 2.1: #3882

#3882:
- An XML comment node as parameter in widget.xml fails with fatal error
- Fixed also for comments in containers node

(cherry picked from commit 51da3d4)

MAGETWO-65436: [GitHub][PR] #3882 #8772

(cherry picked from commit 03c9c20)

Author: madotto

app/code/Magento/Widget/Model/Config/Converter.php

@@ -44,7 +44,7 @@ public function convert($source)
                     case 'parameters':
                         /** @var $parameter \DOMNode */
                         foreach ($widgetSubNode->childNodes as $parameter) {
-                            if ($parameter->nodeName === '#text') {
+                            if ($parameter->nodeName === '#text' || $parameter->nodeName === '#comment') {
                                 continue;
                             }
                             $subNodeAttributes = $parameter->attributes;
@@ -57,7 +57,7 @@ public function convert($source)
                             $widgetArray['supported_containers'] = [];
                         }
                         foreach ($widgetSubNode->childNodes as $container) {
-                            if ($container->nodeName === '#text') {
+                            if ($container->nodeName === '#text' || $container->nodeName === '#comment') {
                                 continue;
                             }
                             $widgetArray['supported_containers'] = array_merge(

app/code/Magento/Widget/Test/Unit/Model/_files/widget.xml

@@ -11,6 +11,7 @@
         <label translate="true">Orders and Returns</label>
         <description translate="true">Orders and Returns Search Form</description>
         <parameters>
+            <!-- Comment added to check fix to https://github.com/magento/magento2/issues/3882 -->
             <parameter name="title" xsi:type="text" visible="false">
                 <label translate="true">Anchor Custom Title</label>
             </parameter>
@@ -54,6 +55,7 @@
             </parameter>
         </parameters>
         <containers>
+            <!-- Comment added to check fix to https://github.com/magento/magento2/issues/3882 -->
             <container name="left">
                 <template name="default" value="default_template" />
             </container>