Navigation drawer TextView Nullpointer exception - android-studio

Question

I have a TextView in the navigation drawer of my app. When initially loading the relevant xml, the TextView (displaying users name and surname) runs fine without an error. However, after navigating to the next activity and returning immediately back to the previous activity the app crashes with a NullPointerException. Why does it load it successfully initially but crashes when navigating back? I have put in a null check but this just sets the TextView's to blank but prevents the crashing.
My navigation header xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<ImageView
android:id="#+id/imageView2"
android:layout_width="match_parent"
android:layout_height="117dp"
android:paddingLeft="10dp"
android:paddingRight="10dp"
app:srcCompat="#drawable/logo" />
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal">
<TextView
android:id="#+id/textView15"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:paddingLeft="20dp"
android:textSize="24sp" />
<TextView
android:id="#+id/textView40"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="24sp" />
</LinearLayout>
</LinearLayout>
Part of the layout xml where i am referencing the navigation header:
<com.google.android.material.navigation.NavigationView
android:id="#+id/mydeals_seller_nav_view"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_gravity="start"
app:headerLayout="#layout/navigation_header"
android:fitsSystemWindows="true"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:menu="#menu/seller_menu"/>
Referencing the TextView in the activity:
docref = FirebaseFirestore.getInstance()
docref.collection("Users").whereEqualTo("uid", userid).get()
.addOnSuccessListener { value->
if (value != null ) {
for (document in value!!) {
val user_name = document.getString("name")!!
val user_surname = document.getString("surname")!!
if(textView15 != null || textView40 != null){
menu_name = findViewById(R.id.textView15)!!
menu_surname = findViewById(R.id.textView40)!!
menu_name.setText("$user_name ")
menu_surname.setText(user_surname)
}
}
} else {
Log.d("Firestore_error", "No Data")
}
}
Do I need a separate navigation header and TextView for each activity?
Edit
Testing this further on API 22, this issue doesnt present itself. Any suggestions?

Answer 1

Instead of calling the findViewById on the activity view you should call it on the header view like so:
docref.collection("Users").whereEqualTo("uid", userid).get()
.addOnSuccessListener { value->
if (value != null ) {
for (document in value!!) {
val user_name = document.getString("name")!!
val user_surname = document.getString("surname")!!
// This gets the first header that is attached to the
// navigation view
val header = mydeals_seller_nav_view.getHeaderView(0)
menu_name = header.findViewById(R.id.textView15)!!
menu_surname = header.findViewById(R.id.textView40)!!
menu_name.setText("$user_name ")
menu_surname.setText(user_surname)
}
}
} else {
Log.d("Firestore_error", "No Data")
}
}
Hope these fixies your issue

Answer 2

menu_name = findViewById(R.id.textView15)!!
menu_surname = findViewById(R.id.textView40)!!
menu_name.setText("$user_name ")
menu_surname.setText(user_surname)
Replace those lines with these:
val headerView = mydeals_seller_nav_view.getHeaderView(0)
val menu_name = headerView.textView15
val menu_surname = headerView.textView40
menu_name.setText("$user_name ")
menu_surname.setText(user_surname)

Answer 3

Please try this...
You have to store that values in prefernces and set preferences values to textview or initialize firbase.instance in onResume() method.

Answer 4

After lots of trial and error its seems like the error was a Firestore query issue. based on my query above i was using a loop where userid id matched the authenticated user. for some reason when moving between activities it resulted in a null pointer. i have no idea why but changing the query above to a single read as below fixed the issue:
docref = FirebaseFirestore.getInstance()
docref.collection("Users").document(userid).get()
.addOnSuccessListener { value->
if (value != null ) {
val user_name = value.getString("name")!!
val user_surname = value.getString("surname")!!
if(textView15 != null || textView40 != null){
menu_name = findViewById(R.id.textView15)!!
menu_surname = findViewById(R.id.textView40)!!
menu_name.setText("$user_name ")
menu_surname.setText(user_surname)
}
} else {
Log.d("Firestore_error", "No Data")
}
}
Just like that, no more NullPointerException. it would be great if a Firestore expert could explain why this was happening.