FATAL EXCEPTION java.lang.NullPointerException: null cannot be cast to non-null type kotlin.String

Question

I try to make my first app, but ı have got the error message. The error message is:

2020-09-17 11:18:17.846 30034-30132/com.ibrahimkiceci.simplynoteapp W/i.simplynoteap: Accessing hidden method Lsun/misc/Unsafe;->getInt(Ljava/lang/Object;J)I (greylist, linking, allowed)

2020-09-17 11:18:18.017 30034-30034/com.ibrahimkiceci.simplynoteapp D/AndroidRuntime: Shutting down VM 2020-09-17 11:18:18.020 30034-30034/com.ibrahimkiceci.simplynoteapp E/AndroidRuntime: FATAL EXCEPTION: main Process: com.ibrahimkiceci.simplynoteapp, PID: 30034 java.lang.NullPointerException: null cannot be cast to non-null type kotlin.String at com.ibrahimkiceci.simplynoteapp.ListViewActivity$getDataFromFirestore$1.onEvent(ListViewActivity.kt:114) at com.ibrahimkiceci.simplynoteapp.ListViewActivity$getDataFromFirestore$1.onEvent(ListViewActivity.kt:19) at com.google.firebase.firestore.Query.lambda$addSnapshotListenerInternal$2(Query.java:1142) at com.google.firebase.firestore.Query$$Lambda$3.onEvent(Unknown Source:6) at com.google.firebase.firestore.core.AsyncEventListener.lambda$onEvent$0(AsyncEventListener.java:42) at com.google.firebase.firestore.core.AsyncEventListener$$Lambda$1.run(Unknown Source:6) at android.os.Handler.handleCallback(Handler.java:883) at android.os.Handler.dispatchMessage(Handler.java:100) at android.os.Looper.loop(Looper.java:214) at android.app.ActivityThread.main(ActivityThread.java:7356) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 2020-09-17 11:18:18.086 30034-30034/com.ibrahimkiceci.simplynoteapp I/Process: Sending signal. PID: 30034 SIG: 9

When ı clicked my sign ın button, the app is closed, I shared the code screen,

How can ı fix it? Thank you!

class ListViewActivity : AppCompatActivity() { private lateinit var auth: FirebaseAuth private lateinit var db : FirebaseFirestore

var titleTextFromFB : ArrayList<String> = ArrayList()
var imageFromFB : ArrayList<String> = ArrayList()
var adapter: NoteAdapter? = null


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

    auth = FirebaseAuth.getInstance()
    db = FirebaseFirestore.getInstance()

    getDataFromFirestore()

    // recyclerview

    var layoutManager = LinearLayoutManager(this)
    recyclerView.layoutManager = layoutManager
    adapter = NoteAdapter(titleTextFromFB, imageFromFB)

    recyclerView.adapter = adapter

}

override fun onCreateOptionsMenu(menu: Menu?): Boolean {

    val menuInflater = menuInflater
    menuInflater.inflate(R.menu.add_note, menu)

    return super.onCreateOptionsMenu(menu)
}

override fun onOptionsItemSelected(item: MenuItem): Boolean {

    if (item.itemId == R.id.add_note) {
        // Take Notes Activity
        val intent = Intent(applicationContext, TakeNotesActivity::class.java)
        startActivity(intent)

    } else if (item.itemId == R.id.log_out) {

        val alert = AlertDialog.Builder(this)

        alert.setTitle("Log out")
        alert.setMessage("Are you sure to logout from the app ?")
        alert.setPositiveButton("Yes") {dialog, which ->

            auth.signOut()
            val intent = Intent(applicationContext, MainActivity::class.java)
            startActivity(intent)
            finish()
        }

        alert.setNegativeButton("No") {dialog, which ->

        }

        alert.show()


    }


    return super.onOptionsItemSelected(item)
}

// get data from firestore

fun getDataFromFirestore() {

    db.collection("Notes").orderBy("date", Query.Direction.DESCENDING).addSnapshotListener { snapshot, exception ->

        if (exception != null) {

            // If there is a error ,

            Toast.makeText(applicationContext, exception.localizedMessage.toString(), Toast.LENGTH_LONG).show()


        } else {

            if (snapshot != null) {

                if (!snapshot.isEmpty) {

                    val documents = snapshot.documents
                    for (document in documents) {

                        val userEmail = document.get("userEmail") as String
                        val noteTitle = document.get("noteTitle") as String
                        val yourNote = document.get("yourNote") as String
                        val downloadUrl = document.get("downloadUrl") as String
                        val timestamp = document.get("date") as Timestamp
                        val date = timestamp.toDate()

                        titleTextFromFB.add(noteTitle)
                        imageFromFB.add(downloadUrl)

                        adapter!!.notifyDataSetChanged()




                    }
                }
            }


        }


      }



  }




}

Answer 1

in your for loop

for (document in documents) {

                    val userEmail = document.get("userEmail") as String
                    val noteTitle = document.get("noteTitle") as String
                    val yourNote = document.get("yourNote") as String
                    val downloadUrl = document.get("downloadUrl") as String
                    val timestamp = document.get("date") as Timestamp
                    val date = timestamp.toDate()

                    titleTextFromFB.add(noteTitle)
                    imageFromFB.add(downloadUrl)

                    adapter!!.notifyDataSetChanged()




                }

one (or more) of the fields are null and you are trying to cast them to a not null String object! so in this case kotlin throws a runtime exception. you should cast it to String? instead of String: like so :

val field = document.get("field") as String?

or check its nullablity :

val field = if (document.get("field") != null) document.get("field") as String
    else ""

Answer 2

The problem was solved when I deleted and reinstalled my database.

Answer 3

As I have understood, your errors are generating from this code block:

val userEmail = document.get("userEmail") as String
val noteTitle = document.get("noteTitle") as String
val yourNote = document.get("yourNote") as String
val downloadUrl = document.get("downloadUrl") as String

Here one of the variables from document is coming as null which cannot be cast to non-nullable String type in kotlin.

If you are not sure of the fields which can come null, write code like this:

val userEmail : String? = document.get("userEmail") as? String
val noteTitle : String?  = document.get("noteTitle") as? String
val yourNote : String? = document.get("yourNote") as? String
val downloadUrl : String? = document.get("downloadUrl") as? String

For more clarity please check Safe(nullable)-Cast operator in kotlin docs