ui: add a default value for customColumns (#6462)

Fixes #6459

(cherry picked from commit c6f12f3)
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

Author: Hoang Nguyen

ui/src/store/modules/user.js

@@ -58,7 +58,8 @@ const user = {
     darkMode: false,
     defaultListViewPageSize: 20,
     countNotify: 0,
-    logoutFlag: false
+    logoutFlag: false,
+    customColumns: {}
   },
 
   mutations: {