Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()'

Question

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' on a null object reference

when I am trying to run app it automatically crashing showing an error

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' on a null object reference
    at etranspo.ph.MainActivity.collectOldImages(MainActivity.java:239)
    at etranspo.ph.MainActivity.lambda$onCreate$1$etranspo-ph-MainActivity(MainActivity.java:118)
    at etranspo.ph.MainActivity$$ExternalSyntheticLambda2.onClick(Unknown Source:2)
    at android.view.View.performClick(View.java:7585)
    at android.view.View.performClickInternal(View.java:7541)
    at android.view.View.access$3900(View.java:842)
    at android.view.View$PerformClick.run(View.java:28875)
    at android.os.Handler.handleCallback(Handler.java:938)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:261)
    at android.app.ActivityThread.main(ActivityThread.java:8279)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:632)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1049)

error pointing to

DatabaseReference imageListReference = FirebaseDatabase
            .getInstance()
            .getReference("profile_images")
            .child(firebaseUser.getUid());

Whole Code

private void collectOldImages()
{
    DatabaseReference imageListReference = FirebaseDatabase
            .getInstance()
            .getReference("profile_images")
            .child(firebaseUser.getUid());

    imageListReference.addValueEventListener(new ValueEventListener()
    {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot)
        {
            imagesList.clear();
            for(DataSnapshot snapshot : dataSnapshot.getChildren())
            {
                imagesList.add(snapshot.getValue(ImagesList.class));
            }

            imagesRecyclerAdapter.notifyDataSetChanged();
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError)
        {
            Toast.makeText(MainActivity.this, databaseError.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });
}