android - 尝试获取LiveData时数据库查询抛出KotlinNullPointerException

我正在尝试在kotlin / android中创建 session 室数据库，但是当我尝试从数据库中获取LiveData时却得到了KotlinNullPointerException。我有两个表Car和Refueling，从一个片段中我可以毫无问题地获取Car数据，但是在第二个片段中，尝试获取Refueling LiveData时出现错误。我找不到这两个片段/ DAO之间的任何区别。下面是一个简化的代码。
加油数据类:

@Entity(tableName = "refueling_table", foreignKeys = [ForeignKey(entity = Car::class,
    parentColumns = arrayOf("carID"),
    childColumns = arrayOf("carID"),
    onDelete = ForeignKey.CASCADE)]
)
data class Refueling(
    @PrimaryKey(autoGenerate = true)
    val refuelingID: Long = 0L,

    @ColumnInfo(name = "carID")
    val carID: Long = 0L,
)

加油DAO:

@Dao
interface RefuelingDAO
{
    @Query("SELECT * FROM refueling_table ORDER BY refuelingID DESC")
    fun getAll(): LiveData<List<Refueling>>
}

应用程序数据库:

@Database(entities = [Car::class, Refueling::class], version = 1, exportSchema = false)
abstract class AppDatabase : RoomDatabase()
{
    abstract val carDAO: CarDAO
    abstract val refuelingDAO: RefuelingDAO

    companion object
    {
        @Volatile
        private var INSTANCE: AppDatabase? = null

        fun getInstance(context: Context): AppDatabase
        {
            synchronized(this)
            {
                var instance = INSTANCE
                if (instance == null)
                {
                    instance = Room.databaseBuilder(
                        context.applicationContext,
                        AppDatabase::class.java,
                        "app_database"
                    ).fallbackToDestructiveMigration().build()
                    INSTANCE = instance
                }
                return instance
            }
        }
    }
}

在片段中创建ViewModel:

val arguments = CarFragmentArgs.fromBundle(requireArguments())
val application = requireNotNull(this.activity).application
val dataSource = AppDatabase.getInstance(application).refuelingDAO
val viewModelFactory = CarFragmentVMFactory(dataSource, arguments.carID, application)

viewModel = ViewModelProvider(this, viewModelFactory).get(CarFragmentVM::class.java)

binding.carViewModel = viewModel
binding.lifecycleOwner = this
binding.buttonTest.setOnClickListener {
    viewModel.listAll()
}

ViewModel:

class CarFragmentVM(
    private val database: RefuelingDAO,
    private val carID: Long,
    application: Application
) :
        AndroidViewModel(application)
{
    val refueling = database.getAll()

    fun listAll()
    {
        refueling.value!!.forEach { \\ IN THIS PLACE I GET ERROR
            Log.d(it)
        }
    }
}

当我点击“buttonTest”时，我得到了KotlinNullPointerException。正如我说的那样，我在另一个片段中有相同的代码(我只是找不到任何区别)，并且可以正常工作，但事实并非如此。谁能看到我的问题在哪里，或者提供任何线索我应该在哪里寻找？
Logcat:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.fuelmanager, PID: 22828
    kotlin.KotlinNullPointerException
        at com.example.fuelmanager.ui.car.CarFragmentVM.listAll(CarFragmentVM.kt:44)
        at com.example.fuelmanager.ui.car.CarFragment$onCreateView$1.onClick(CarFragment.kt:44)
        at android.view.View.performClick(View.java:7125)
        at android.view.View.performClickInternal(View.java:7102)
        at android.view.View.access$3500(View.java:801)
        at android.view.View$PerformClick.run(View.java:27336)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

第44行是

refueling.value!!.forEach { \\ IN THIS PLACE I GET ERROR
            Log.d(it)
        }

最佳答案

getAll()返回LiveData。实际的数据库查询是在后台线程上完成的。这将需要一些时间才能完成。同时，value是null。但是，您假设它将立即完成。因此，您尝试立即使用value，并使用NullPointerException崩溃。
请observe() LiveData。