安卓应用加载报错：调用ActionBar.setTitle出现空对象引用异常

报错栈信息：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
at com.example.android.bettingapp.DashboardActivity.onCreate(DashboardActivity.kt:27)

该异常是setupActionBarWithNavController方法内部调用ActionBar.setTitle时，ActionBar实例为null导致，结合代码有两个核心触发点：

• 未通过ViewBinding正确获取Toolbar实例：代码中setSupportActionBar(myToolBar)直接使用myToolBar，没有通过已初始化的dashboardBinding获取布局中的Toolbar控件，导致传入setSupportActionBar的参数为null，后续supportActionBar全局为空。
• 逻辑顺序冲突：代码调用supportActionBar?.hide()后仍执行setupActionBarWithNavController，该方法默认会操作ActionBar更新标题，即使前面Toolbar不为空，hide操作也可能导致后续调用异常。

方案一：保留ActionBar配合导航使用 #

1. 修正Toolbar引用，替换setSupportActionBar(myToolBar)为：

setSupportActionBar(dashboardBinding.myToolBar)

2. 删除多余的supportActionBar?.hide()代码，最终调整后的onCreate相关代码如下：

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    dashboardBinding = ActivityDashboardBinding.inflate(layoutInflater)
    setContentView(dashboardBinding.root)
    // 修正Toolbar引用
    setSupportActionBar(dashboardBinding.myToolBar)
    val navView: BottomNavigationView = dashboardBinding.bottomNavView
    val navController = findNavController(R.id.nav_host_fragment)
    val appBarConfiguration = AppBarConfiguration(
        setOf(
            R.id.homeFragment,
            R.id.newBoastFragment,
            R.id.myBoastFragment,
            R.id.profileFragment
        )
    )
    setupActionBarWithNavController(navController, appBarConfiguration)
    navView.setupWithNavController(navController)
    supportActionBar?.setDisplayHomeAsUpEnabled(true)
    supportActionBar?.setHomeButtonEnabled(true)
}

方案二：完全隐藏ActionBar #

如果不需要展示顶部ActionBar，直接删除所有ActionBar相关操作即可，仅保留底部导航绑定逻辑：

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    dashboardBinding = ActivityDashboardBinding.inflate(layoutInflater)
    setContentView(dashboardBinding.root)
    val navView: BottomNavigationView = dashboardBinding.bottomNavView
    val navController = findNavController(R.id.nav_host_fragment)
    navView.setupWithNavController(navController)
}

内容的提问来源于stack exchange，提问作者Jaanu