Java 错误：应用程序不断崩溃，无法加载

我对编程有点陌生，不确定我的代码哪里做错了

错误如下所示

     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.bottomnavigation.BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView$OnNavigationItemSelectedListener)' on a null object reference
        at com.example.cleverreality.MainActivity.onCreate(MainActivity.java:42)

这是我一直在处理的片段代码，第二行有错误

        BottomNavigationView bottomNav = findViewById(R.id.bottom_navigation);
        bottomNav.setOnNavigationItemSelectedListener(navListener);
    }
    private BottomNavigationView.OnNavigationItemSelectedListener navListener =
            new BottomNavigationView.OnNavigationItemSelectedListener() {
                @Override
                public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                    Fragment selectedFragment = null;

                    switch (item.getItemId()) {
                        case R.id.home:
                            selectedFragment = new HomeFragment();
                            break;
                        case R.id.settings:
                            selectedFragment = new SettingsFragment();
                            break;
                        case R.id.devices:
                            selectedFragment = new DevicesFragment();
                            break;
                    }
                getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
                        selectedFragment).commit();

                    return true;
                }
            };

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