无法理解nullpointerexception

Question

我开发一个登录和注册程序与android，php和mysql。我得到这个例子可以请任何人帮助我。该例程在-if（json.getString（KEY_SUCCESS）！= null）

上

08-21 15:30:07.560: E/AndroidRuntime(9712): FATAL EXCEPTION: main
08-21 15:30:07.560: E/AndroidRuntime(9712): java.lang.NullPointerException
08-21 15:30:07.560: E/AndroidRuntime(9712):     at com.example.ritkigatestadatabas.RegisterActivity$1.onClick(RegisterActivity.java:65)
08-21 15:30:07.560: E/AndroidRuntime(9712):     at android.view.View.performClick(View.java:4191)
08-21 15:30:07.560: E/AndroidRuntime(9712):     at android.view.View$PerformClick.run(View.java:17229)

这是注册活动类

public class RegisterActivity extends Activity {
        Button btnRegister;
        Button btnLinkToLogin;
        EditText inputFullName;
        EditText inputEmail;
        EditText inputPassword;
        TextView registerErrorMsg;

        // JSON Response node names
        private static String KEY_SUCCESS = "success";
        private static String KEY_ERROR = "error";
        private static String KEY_ERROR_MSG = "error_msg";
        private static String KEY_UID = "uid";
        private static String KEY_NAME = "name";
        private static String KEY_EMAIL = "email";
        private static String KEY_CREATED_AT = "created_at";

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.register);

            // Importing all assets like buttons, text fields
            inputFullName = (EditText) findViewById(R.id.registerName);
            inputEmail = (EditText) findViewById(R.id.registerEmail);
            inputPassword = (EditText) findViewById(R.id.registerPassword);
            btnRegister = (Button) findViewById(R.id.btnRegister);
            btnLinkToLogin = (Button) findViewById(R.id.btnLinkToLoginScreen);
            registerErrorMsg = (TextView) findViewById(R.id.register_error);

            // Register Button Click event
            btnRegister.setOnClickListener(new View.OnClickListener() {        
                public void onClick(View view) {
                    String name = inputFullName.getText().toString();
                    String email = inputEmail.getText().toString();
                    String password = inputPassword.getText().toString();
                    UserFunctions userFunction = new UserFunctions();
                    Log.i("funkar","skickar" + name);
                    JSONObject json = userFunction.registerUser(name, email, password);
                    Log.i("funkar","får tillbaka json");


                    // check for login response
                    try {
                     HERE IS THE EXEPTION-----   if (json.getString(KEY_SUCCESS) != null) {
                            Log.i("funkar",""+KEY_SUCCESS);
                            registerErrorMsg.setText("");
                            String res = json.getString(KEY_SUCCESS);
                            if(Integer.parseInt(res) == 1){
                                // user successfully registred
                                // Store user details in SQLite Database
                                DatabaseHandler db = new DatabaseHandler(getApplicationContext());
                                Log.i("funkar","sparar i datasbasen");
                                JSONObject json_user = json.getJSONObject("user");

                                // Clear all previous data in database
                                userFunction.logoutUser(getApplicationContext());
                                db.addUser(json_user.getString(KEY_NAME), json_user.getString(KEY_EMAIL), json.getString(KEY_UID), json_user.getString(KEY_CREATED_AT));                       
                                // Launch Dashboard Screen
                                Intent dashboard = new Intent(getApplicationContext(), DashboardActivity.class);
                                // Close all views before launching Dashboard
                                dashboard.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                                startActivity(dashboard);
                                // Close Registration Screen
                                finish();
                            }else{
                                // Error in registration

                                registerErrorMsg.setText("Error occured in registration");
                            }
                        }
                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                }
            });

            // Link to Login Screen
            btnLinkToLogin.setOnClickListener(new View.OnClickListener() {

                public void onClick(View view) {
                    Intent i = new Intent(getApplicationContext(),
                            LoginActivity.class);
                    startActivity(i);
                    // Close Registration View
                    finish();
                }
            });
        }
    }