clear button get exception when EditText is empty

the problem is in here
back.setOnClickListener(new OnClickListener() {

```
        @Override
        public void onClick(View arg0) {

            String lock = unlock.getText().toString();
            lock = lock.substring(0, lock.length() - 1);
            unlock.setText(lock);

        }
    });
```

it will fix with try catch ;)
