Null Exception when performing where()

I am using realm and I have the code

 realm = Realm.getDefaultInstance();
        realm.beginTransaction();
        RealmQuery<LogEntry> query = realm.where(LogEntry.class);

To query for results. Occasionally however this throws

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'io.realm.RealmQuery io.realm.Realm.where(java.lang.Class)' on a null object reference

Error. How can I prevent this?