java.lang.IllegalStateException: This Realm instance has already been closed, making it unusable

I am trying to get value from my realm db something like this:

public static List<MailData> getAllMailList() {
    Realm realm = Realm.getDefaultInstance();
    List<MailData> mails = new ArrayList<>();
    try {
        mails = realm.where(MailData.class)
                .sort("receiveDate", Sort.DESCENDING)
                .findAll();
        return mails;
    }catch (Exception e){
        e.printStackTrace();
    }finally {
        realm.close();
    }
    return mails;
}

or I have tried this approach as well:

try (Realm realm = Realm.getDefaultInstance()) {
        return realm.where(MailData.class)
                .sort("receiveDate", Sort.DESCENDING)
                .findAll();
    }

Now, I am calling this in my activity class to read the data like this :

for (MailData data:
                getAllMailList()) {
            Log.d("Data is:", data.getSubject());
        }

While I run the code, showing me this error :

java.lang.IllegalStateException: This Realm instance has already been closed, making it unusable.

If i remove realm.close() from finally block, can't delete realm db while logout. Showing me this error :

java.lang.IllegalStateException: It's not allowed to delete the file associated with an open Realm. Remember to close() all the instances of the Realm before deleting its file:

To clear Realm data while logout, code is:

try(Realm realm = Realm.getDefaultInstance()) {
            realm.executeTransaction(realm1 -> {
                Realm.deleteRealm(realm1.getConfiguration());
                realm1.deleteAll();
            });
        }catch (Exception e){
            e.printStackTrace();
        }

So my questions are:

1. How to access realm data after closing the realm instance?
2. How to delete realm all data while logout?