CRM online - Form script error on save after applying Update rollup 12

Hi  Everyone

We are experiencing major script error in our code base on javascript ( on save event) of entity after we upgraded our   Crm Online Customziation to  UR 12.

Same script was working till UR 11 in all IE versions.

Sample Code snippet for preventing save of Form (in case, Validation rule is breached) written on Form (on save event)

event.returnValue = false; return false;

I have also tried  alternate method suggested on blogs for preventing form Save event ,even these  are not working (executionObj.getEventArgs().preventDefault()) or ExecutionObj.getEventArgs().isDefaultPrevented();

with executionObj being context name on UR 12 CRMonline environment.

Please find attached error.

i need to know,why this has stopped working or any workaround  to solve this issue.

Sumit kumar