How to fix error: The operation cannot be completed because the DbContext has been disposed.

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Replace

        public IQueryable getGroups()
        {
            using (CompanyTestEntities db = new CompanyTestEntities())
            {
                var result = db.GroupMasters;
                return result;
            }
        }
With

        public List<GroupMaster> getGroups()
        {
            using (CompanyTestEntities db = new CompanyTestEntities())
            {
                var result = db.GroupMasters.ToList();
                return result;
            }
        }