Fix Cannot read property '_buttons' of undefined

The buttons should only be created if the items_table exists

JIRA INFRA-584

app/static/js/items.js

@@ -93,18 +93,20 @@ $(document).ready(function() {
     ]
   });
 
-  new $.fn.dataTable.Buttons(items_table, {
-    buttons: [
-      {
-        text: '<span class="oi oi-data-transfer-download" title="Export to excel file" aria-hidden="true"></span> Excel',
-        className: "btn-outline-secondary",
-        action: function ( e, dt, node, conf ) {
-          download_excel();
+  if( $("#items_table").length ) {
+    new $.fn.dataTable.Buttons(items_table, {
+      buttons: [
+        {
+          text: '<span class="oi oi-data-transfer-download" title="Export to excel file" aria-hidden="true"></span> Excel',
+          className: "btn-outline-secondary",
+          action: function ( e, dt, node, conf ) {
+            download_excel();
+          }
         }
-      }
-    ]
-  });
+      ]
+    });
 
-  items_table.buttons().container().appendTo("#items_table_wrapper .col-md-3:eq(0)");
+    items_table.buttons().container().appendTo("#items_table_wrapper .col-md-3:eq(0)");
+  }
 
 });