Not getting Debug log of my apex class which is using QuickActionDefaultsHandler interface

Question

I am trying to set default 'From' in Send email option in case feed. for this i am using below code but it seems not be running and i am not getting and debug log. NOTE: in support setting i have added this apex class to be run but still it is not running.

    global class EmailPublisherLoader implements QuickAction.QuickActionDefaultsHandler {
    // Empty constructor
    global EmailPublisherLoader() {
    }

    // The main interface method
    global void onInitDefaults(QuickAction.QuickActionDefaults[] defaults) {
        QuickAction.SendEmailQuickActionDefaults sendEmailDefaults = null;

        // Check if the quick action is the standard Case Feed send email action
        for (Integer j = 0; j < defaults.size(); j++) {
            if (defaults.get(j) instanceof QuickAction.SendEmailQuickActionDefaults && 
               defaults.get(j).getTargetSObject().getSObjectType() == 
                   EmailMessage.sObjectType && 
               defaults.get(j).getActionName().equals('Case.Email') && 
               defaults.get(j).getActionType().equals('Email')) {
                   sendEmailDefaults = 
                       (QuickAction.SendEmailQuickActionDefaults)defaults.get(j);
                   break;
            }
        }
        system.debug('Defaults---->'+sendEmailDefaults); 
        if (sendEmailDefaults != null) {
            Case c = [SELECT Status,Reason,ownerid FROM Case 
                      WHERE Id=:sendEmailDefaults.getContextId()];

            EmailMessage emailMessage = (EmailMessage)sendEmailDefaults.getTargetSObject();    
            // Set bcc address to make sure each email goes for audit
            emailMessage.FromAddress = getFromAddress();
        }
    }

    private String getFromAddress(/*String reason*/) {
       /* if (reason != null && reason.equals('Technical')) 
            { return 'support_technical@mycompany.com'; } 
        else if (reason != null && reason.equals('Billing')) 
            { return 'support_billing@mycompany.com'; } 
        else { return 'support@mycompany.com'; }*/
        return 'testscaletronix@welchallyn.com';
    } 
}