我不知道我的代码有什么问题

这是我的片段课

package com.example.gandi.symanlub;


/**
 * A simple {@link Fragment} subclass.
 */
public class Reminder extends Fragment {

    Button btnubah, btnkeluar;
    SessionManager session;

    View rootview;



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        rootview = inflater.inflate(R.layout.fragment_reminder, container, false);

        btnkeluar = (Button)rootview.findViewById(R.id.btnlogout);
        btnkeluar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                session.logoutUser();
            }
        });

        return rootview;

    }

}

这是我的SessionManager.java

package com.example.gandi.symanlub;

@SuppressLint("CommitPrefEdits")
public class SessionManager {
    // Shared Preferences
    SharedPreferences pref;

    // Editor for Shared preferences
    SharedPreferences.Editor editor;

    // Context
    Context _context;

    // Shared pref mode
    int PRIVATE_MODE = 0;

    // nama sharepreference
    private static final String PREF_NAME = "Sesi";

    // All Shared Preferences Keys
    private static final String IS_LOGIN = "IsLoggedIn";
    public static final String KEY_EMAIL = "email";
    public static final String KEY_PASS = "pass";



    // Constructor
    public SessionManager(Context context){
        this._context = context;
        pref = _context.getSharedPreferences(PREF_NAME, PRIVATE_MODE);
        editor = pref.edit();
    }

    /**
     * Create login session
     * */
    public void createLoginSession(String email, String pass){
        // Storing login value as TRUE
        editor.putBoolean(IS_LOGIN, true);

        editor.putString(KEY_EMAIL, email);
        editor.putString(KEY_PASS, pass);
        editor.commit();
    }

    /**
     * Check login method wil check user login status
     * If false it will redirect user to login page
     * Else won't do anything
     * */
    public void checkLogin(){
        // Check login status
        if(!this.isLoggedIn()){
            Intent i = new Intent(_context, Login.class);
            i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
            i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            _context.startActivity(i);
            //((Activity)_context).finish();
        }

    }

    /**
     * Get stored session data
     * */
    public HashMap<String, String> getUserDetails(){
        HashMap<String, String> user = new HashMap<String, String>();

        user.put(KEY_EMAIL, pref.getString(KEY_EMAIL, null));
        user.put(KEY_PASS, pref.getString(KEY_PASS, null));

        return user;
    }

    /**
     * Clear session details
     * */
    public void logoutUser(){
        editor.clear();
        editor.commit();
    }

    public void hapussesi(){
        editor.clear();
        editor.commit();
    }

    public boolean isLoggedIn(){
        return pref.getBoolean(IS_LOGIN, false);
    }
}

运行项目时出现的错误：

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ void
com.example.gandi.symanlub.SessionManager.logoutUser（）”