完整错误堆栈跟踪：

04-05 16：01：34.265 18746-18746 /？ E / AndroidRuntime：FATAL EXCEPTION：main进程：com.example.mrathi.livedataviewmodel，PID：18746 java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.mrathi.livedataviewmodel / com.example.mrathi.livedataviewmodel.MainActivity在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2726）上的android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2665）的android.app.ActivityThread.-wrap12（ActivityThread.java）处的kotlin.KotlinNullPointerException android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1477）在android.app.Handler.dispatchMessage（Handler.java:102）android.app.Looper.loop（Looper.java:154）android.app .ActivityThread.main（ActivityThread.java:6119）位于com.android的com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:886）的java.lang.reflect.Method.invoke（Native Method） .internal.os.ZygoteInit.main（ZygoteInit.java:776）引起：com.example.mrathi.li的kotlin.KotlinNullPointerException android.app.ActivityThread.performLaunchActivity上android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1118）的android.app.Activity.performCreate（Activity.java:6679）上的vedataviewmodel.MainActivity.onCreate（MainActivity.kt：40） （ActivityThread.java:2618）位于android.app.A活动时的android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2726）android.app.ActivityThread $ H.handleMessage（ActivityThread.java）上的android.app.ActivityThread.-wrap12（ActivityThread.java） ：1477）在Android.os.Handler.dispatchMessage（Handler.java:102）的android.app.Looper.loop（Looper.java:154）在android.app.ActivityThread.main（ActivityThread.java:6119）在java位于com.android.internal.os.ZygoteInit的com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:886）中的.lang.reflect.Method.invoke（Native Method）（ZygoteInit.java： 776）

这是我的带有api的数据库类（在Kotlin中使用MVVM模型）

@Database(entities = arrayOf(BorrowModel::class), version = 1)
abstract class AppDatabase : RoomDatabase() {

    companion object {
        var INSTANCE: AppDatabase? = null

        fun getDatabase(context: Context): AppDatabase {
            if (INSTANCE == null) {
                **INSTANCE = Room.databaseBuilder(context.applicationContext, AppDatabase::class.java, "borrow_db")
                        .build()** through target invocation exception
            }
            return INSTANCE as AppDatabase
        }
    }
    abstract fun itemAndPersonModel(): BorrowModelDao
}

这是我用于观察UI更新的数据库更改的活动代码：

class MainActivity : AppCompatActivity() , View.OnLongClickListener {

    override fun onLongClick(v: View?): Boolean {
        val borrowModel = v!!.getTag() as BorrowModel
        viewModel!!.deleteItem(borrowModel)
        return true
    }

     var viewModel: BorrowedListViewModel? = null
    //  private var recyclerViewAdapter: RecyclerViewAdapter? = null
    // private var recyclerView: RecyclerView? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        var users = ArrayList<BorrowModel>()
        var obj_adapter = RecyclerViewAdapter(users, this);

        recyclerView.layoutManager = LinearLayoutManager(this, 
        LinearLayout.VERTICAL, false)
        recyclerView.adapter = obj_adapter
        try {
            viewModel = 
          ViewModelProviders.of(this).get(BorrowedListViewModel::class.java)
        }catch ( e :Exception){
            e.toString()
        }


      viewModel!!.getItemAndPersonList()!!.observe(this, object : 
            Observer<List<BorrowModel>?> {
                override fun onChanged(t: List<BorrowModel>?) {
                    Log.v("OnChanged","OnChanged!!")
                    obj_adapter.addItems(t)
            }
        })
    }
}

这是 ViewModel 类：

class BorrowedListViewModel(application: Application) : AndroidViewModel(application) {

    private var itemAndPersonList: LiveData<List<BorrowModel>>? = null
    private var appDatabase: AppDatabase? = null

    init {
        appDatabase = AppDatabase.getDatabase(this.getApplication())

        itemAndPersonList = appDatabase!!.itemAndPersonModel().getAllBorrowedItems()

    }

    fun getItemAndPersonList(): LiveData<List<BorrowModel>>? {
        return itemAndPersonList
    }

    fun deleteItem(borrowModel: BorrowModel) {
        deleteAsyncTask(appDatabase!!).execute(borrowModel)
    }

    class deleteAsyncTask internal constructor(private var db: AppDatabase) : AsyncTask<BorrowModel, Void, Void>() {

        override fun doInBackground(vararg params: BorrowModel): Void? {
            db.itemAndPersonModel().deleteBorrow(params[0])
            return null
        }

    }

}
android kotlin android-room