Do not fail displaying notification when workflow has been deleted

src/Bundle/ChillMainBundle/Resources/views/Workflow/_notification_include.html.twig

@@ -1,11 +1,15 @@
-<h2>
-    {{ 'workflow_'|trans }}
-</h2>
+{% if handler is not null %}
+    <h2>
+        {{ 'workflow_'|trans }}
+    </h2>
 
-<div class="item-row col" style="display: block;">
-    {% include handler.template(entity_workflow) with handler.templateData(entity_workflow)|merge({
-        'description': true,
-        'breadcrumb': true,
-        'add_classes': ''
-    }) %}
-</div>
+    <div class="item-row col" style="display: block;">
+        {% include handler.template(entity_workflow) with handler.templateData(entity_workflow)|merge({
+            'description': true,
+            'breadcrumb': true,
+            'add_classes': ''
+        }) %}
+    </div>
+{% else %}
+    <h2>{{ 'workflow.deleted_title'|trans }}</h2>
+{% endif %}