Sign Up in Parse shows error : com.parse.ParseUser.signUpInBackground()' on a null object reference

Question

Complete Error:

"Attempt to invoke virtual method 'bolts.Task com.parse.ParseUser.signUpInBackground()' on a null object reference"

I am trying to make a login and sign up button, on clicking it the user must be signed in and if failed then signed up.

Here is my source code:

public void onClickLogInSignUp (View view)
{
    EditText editTextUsername = (EditText) findViewById(R.id.editTextUsername);
    EditText editTextPassword = (EditText) findViewById(R.id.editTextPassword);

    username = editTextUsername.getText().toString();
    password = editTextPassword.getText().toString();

    ParseUser.logInInBackground(username, password, new LogInCallback()
    {

        @Override
        public void done(ParseUser user, ParseException e)
        {
            Log.i("Parse","Trying to log in ");

            if ( e == null )
            {
                Log.i("Parse","Log In Successful");
                Log.i("Parse","Redirecting as "+ ParseUser.getCurrentUser().getUsername());
                Toast.makeText(MainActivity.this, "Logged In", Toast.LENGTH_SHORT).show();
                startActivity(new Intent(getApplicationContext(), UserList.class));
            }
            else
            {
                Log.e("Parse","Log In Failed Error - "+e.toString());
                isLogInSuccessful = false;
                ParseUser.getCurrentUser().signUpInBackground();
                ParseUser.getCurrentUser().saveInBackground(new SaveCallback()
                {
                    @Override
                    public void done(ParseException e)
                    {
                        ParseUser user = new ParseUser();
                        user.setUsername(username);
                        user.setPassword(password);
                        user.signUpInBackground(new SignUpCallback()
                        {
                            @Override
                            public void done(ParseException e)
                            {
                                if (e == null)
                                {
                                    startActivity(new Intent(getApplicationContext(), UserList.class));
                                    Log.i("Parse","Sign Up Successful");
                                    Log.i("Parse","Redirecting as "+ ParseUser.getCurrentUser().getUsername());
                                    Toast.makeText(MainActivity.this, "Signed Up", Toast.LENGTH_SHORT).show();
                                }
                                else
                                {
                                    Log.e("Parse","Can't log in as well as signup.");
                                    Toast.makeText(MainActivity.this, "Can't log in as well as signup.", Toast.LENGTH_SHORT).show();
                                }
                            }
                        });

                    }
                });
            }
        }
    });
}