Attempt to invoke virtual method 'java.lang.String com.parse.ParseUser.getUsername()' on a null object reference

public class Setting extends Activity {

    Button finishButton;
    String usernametxt;

    String FirstNametxt;
    String LastNametxt;
    String Emailtxt;
    String FFirstNametxt;
    String FLastNametxt;
    String FEmailtxt;
    String FMobileNumbertxt;

    EditText username;
    EditText FirstName;
    EditText LastName;
    EditText Email;
    EditText FFirstName;
    EditText FLastName;
    EditText FEmail;
    EditText FMobileNumber;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.setting);

        ParseUser currentUser = ParseUser.getCurrentUser();
        usernametxt = currentUser.getUsername();
        Emailtxt = currentUser.getEmail();
        FirstNametxt = currentUser.get("FirstName").toString();
        LastNametxt = currentUser.get("LastName").toString();
        FFirstNametxt = currentUser.get("FFirstName").toString();
        FLastNametxt = currentUser.get("FLastName").toString();
        FEmailtxt = currentUser.get("FEmail").toString();
        FMobileNumbertxt = currentUser.get("FMobileNumber").toString();

        username = (EditText) findViewById(R.id.Username);
        FirstName = (EditText) findViewById(R.id.FirstName);
        LastName = (EditText) findViewById(R.id.LastName);
        Email = (EditText) findViewById(R.id.Email);
        FFirstName = (EditText) findViewById(R.id.FFirstName);
        FLastName = (EditText) findViewById(R.id.FLastName);
        FEmail = (EditText) findViewById(R.id.FEmail);
        FMobileNumber = (EditText) findViewById(R.id.FMobileNumber);

        username.setText(usernametxt);
        Email.setText(Emailtxt);
        FirstName.setText(FirstNametxt);
        LastName.setText(LastNametxt);
        FFirstName.setText(FFirstNametxt);
        FLastName.setText(FLastNametxt);
        FEmail.setText(FEmailtxt);
        FMobileNumber.setText(FMobileNumbertxt);

        finishButton= (Button)findViewById(R.id.finishbtn);

        finishButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {

                usernametxt = username.getText().toString();
                FirstNametxt = FirstName.getText().toString();
                LastNametxt = LastName.getText().toString();
                Emailtxt = Email.getText().toString();
                FFirstNametxt = FFirstName.getText().toString();
                FLastNametxt = FLastName.getText().toString();
                FEmailtxt = FEmail.getText().toString();
                FMobileNumbertxt = FMobileNumber.getText().toString();

                ParseQuery<ParseObject> query = ParseQuery.getQuery("User");
                query.whereEqualTo("objectId", ParseUser.getCurrentUser().getObjectId());
                query.findInBackground(new FindCallback<ParseObject>() {
                    public void done(List<ParseObject> objects, ParseException e) {
                        if (usernametxt.equals("")  &&
                                LastNametxt.equals("") && Emailtxt.equals("")  &&
                                FirstNametxt.equals("")&& FFirstNametxt.equals("")  &&
                                FLastNametxt.equals("") && FEmailtxt.equals("")  &&
                                FMobileNumbertxt.equals("")) {

                            AlertDialog.Builder builder = new AlertDialog.Builder(Setting.this);

                            AlertDialog dialog = builder.create();
                            dialog.show();

                        } else {

                            //Update user
                            ParseUser user = ParseUser.getCurrentUser();
                            user.put("username", usernametxt);
                            user.put("FirstName", FirstNametxt);
                            user.put("LastName", LastNametxt);
                            user.put("Email", Emailtxt);

                            user.put("FFirstName", FFirstNametxt);
                            user.put("FLastName", FLastNametxt);
                            user.put("FEmail", FEmailtxt);
                            user.put("FMobileNumber", FMobileNumbertxt);

                            user.saveInBackground(new SaveCallback() {

                                @Override
                                public void done(ParseException e) {

                                    if (e == null) {
                                        Intent intent = new Intent(
                                                Setting.this,
                                                MainMenu.class);

                                        Toast.makeText(getApplicationContext(),
                                                "Updated!!",
                                                Toast.LENGTH_SHORT).show();
                                        startActivity(intent);

                                    } else {
                                        Toast.makeText(getApplicationContext(),
                                                "NOT Updated!!", Toast.LENGTH_SHORT)
                                                .show();
                                    }
                                }

                            });

                        }
                    }
                });
            }

        });
    }
}