Fix item-add save (#2346)

Regression from #2339 causing a new item unable to be created / saved.

Signed-off-by: Jimmy Tanagra <jcode@tanagra.id.au>

bundles/org.openhab.ui/web/src/pages/settings/items/item-edit.vue

@@ -171,7 +171,7 @@ export default {
       }
     },
     save () {
-      if (!this.item.editable) return
+      if (!this.editable) return
       if (this.currentTab === 'code') {
         if (!this.fromYaml()) return
       }