Error throwing on checking whether app got notification access or not?

Question

I have an app in which i have to check whether my app got notification acccess or not for this i am using below mention code but this is throwing error on testing on device above api level 23. Pls help me to resolve this issue.

code:-

 ContentResolver cr = getContentResolver();
        String setting = "enabled_notification_listeners";
        String permissionString = Settings.Secure.getString(cr, setting);
        if (permissionString.contains("com.ermo.batt")) {
            intent = new Intent(mContext, CNotificationHomeActivity.class);
            startActivity(intent);
        } else {
            intent = new Intent(mContext, CNotificationManagerWalkThrough.class);
            startActivity(intent);
        }

error:-

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.contains(java.lang.CharSequence)' on a null object reference