在 NavigationUI 上执行命令时出现 NullPointerException

Question

无法找到导致空异常问题的原因。按照指南制作项目，它没有这个问题。使用与 guidr 相同的库版本。

错误信息

Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.navigation.NavController
import androidx.navigation.Navigation
import androidx.navigation.ui.NavigationUI
import androidx.navigation.ui.setupWithNavController
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    private lateinit var navController: NavController
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        navController = Navigation.findNavController(this, R.id.nav_host_fragment)
        bottom_nav.setupWithNavController(navController)


        NavigationUI.setupActionBarWithNavController(this, navController)

    }

    override fun onSupportNavigateUp(): Boolean {
        return NavigationUI.navigateUp( navController, null)
    }
}

Answer 1

setContentView(R.layout.activity_main)
setSupportActionBar(toolbar)
navController = Navigation.findNavController(this, R.id.nav_host_fragment)

你setSupportActionBar(toolbar)在setContentView和之间失踪navController = ...