Enabling and disabling test cases from test suite

I have excel with all test cases for the suite.My req is to disable all testcases from test suite start  up script when the test begins.Based on if the test case is set for run (one excel cloumn would read "yes" for run),i would be enabling the tets cases that are required to be run.

So,I am trying to Enable and Disable test case from Test Suite level.

From Test suite level start up script,
i am trying as below

isDisabled=runner.testSuite.getTestCaseByName(strLabel).isDisabled() 
if (isDisabled!=true) 
{runner.testSuite.getTestCaseByName(TestCaseLabel).disabled=true} 

throws error ---cannot invoke method on null object.

Scenario 2:
Trying to enable all disabled test cases from start up script.

runner.testSuite.getTestCaseByName(strExcelTestCase).disabled=false

again same error.Please help.This is stopping the framework development