As I've mentioned in the original post that I've overriden the OOTB UI Action.I can see my UI action have successfully overriden the OOTB version as I can see custom validation message is getting displayed on the screen. Both UI action and UI page are under application scope of Major incident management. In OOTB UI action I am just giving reference to my custom UI Page like this 

 

 

function openMIMWorkbench() {
    alert('Success');
    var url = "/$custom_workbench.do?sysparm_stack=no&sysparm_sys_id=" + g_form.getUniqueValue();
   
   
    var isPolarisEnabled = isPolaris();
    if (isPolarisEnabled) {
        var polarisPrefixUrl = "/now/nav/ui/classic/params/target/";
        var query = "/$custom_workbench.do?sysparm_stack=no&sysparm_full_theme_support=true&sysparm_use_polaris=true&sysparm_sys_id=" + g_form.getUniqueValue();
        query = encodeURIComponent(query);
        url = polarisPrefixUrl + query;
    }
   
    var gsftFrame = parent.document.getElementById('gsft_main');
    if(!gsftFrame)
            gsftFrame = parent.document.getElementById('testFrame');
    if (gsftFrame) {
        var gsftWindow = gsftFrame.contentWindow || gsftFrame.contentDocument;
        gsftWindow.location = url;
    }
    else {
        parent.location = url;
    }
}