在调用navigate()方法之前，要确保NavController已设置。以下是可能的解决方法之一：

将导航控制器（NavController）分配给Activity或Fragment：

class MyActivity : AppCompatActivity() { // ... override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(layout.main_activity)

    val navController = findNavController(R.id.my_nav_host_fragment)
    setSupportActionBar(findViewById(R.id.toolbar))
    NavigationUI.setupActionBarWithNavController(this, navController)
}

override fun onSupportNavigateUp(): Boolean {
    val navController = findNavController(R.id.my_nav_host_fragment)
    return navController.navigateUp() || super.onSupportNavigateUp()
}

}

确保菜单项与NavController相关联：

override fun onCreateOptionsMenu(menu: Menu): Boolean { menuInflater.inflate(R.menu.my_menu, menu) val navController = findNavController(R.id.my_nav_host_fragment) NavigationUI.onNavDestinationSelected(item, navController) return super.onCreateOptionsMenu(menu) }

当菜单项被选中时，确保切换到正确的目标：

override fun onOptionsItemSelected(item: MenuItem): Boolean { val navController = findNavController(R.id.my_nav_host_fragment) return when (item.itemId) { R.id.action_first_fragment -> { navController.navigate(R.id.firstFragment) true } R.id.action_second_fragment -> { navController.navigate(R.id.secondFragment) true } else -> super.onOptionsItemSelected(item) } }