catching exception in custom error page

 Exception ex = Server.GetLastError().GetBaseException();

        string strexsource;
        string strExdata;
        string strExInnerexception;
        string strExMessage;
        string strExtargetsite;
        
        strexsource = ex.Source.ToString();
        strExdata = ex.Data.ToString();
        strExInnerexception = ex.InnerException.ToString();
        strExMessage = ex.Message.ToString();
        strExtargetsite = ex.TargetSite.ToString();
 
 and I get this error

Object reference not set to ...