Firebase - How to store login session of a User?

Question

I'm using Firebase for Login. I have a user with Email - admin@admin.com , Password 123456789. I want to check if a user is logged in with this account he is authorized to delete the data.

String user = firebaseAuth.getCurrentUser().getUid();
intent.putExtra("name", user);

In MainActivity I'm displaying a toast if the user is same he is authorized.

if(user.equals(intent.getStringExtra("name"))) {
                                Toast.makeText(MainActivity.this, "Authorized", Toast.LENGTH_SHORT).show();
                            } else {
                                Toast.makeText(MainActivity.this, "Not authorized", Toast.LENGTH_SHORT).show();
                            }

It is always returning a null reference.

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference

AdminActivity.java

firebaseAuth.signInWithEmailAndPassword(email,password)
            .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {

                    if(task.isSuccessful()) {
                        Log.d(TAG, "signInWithEmail:success");

                        String user = firebaseAuth.getCurrentUser().getUid();

                        Intent intent = new Intent(AdminActivity.this, MainActivity.class);
                        intent.putExtra("name", user);
                        startActivity(intent);
                        finish();

                    } else {
                        Log.w(TAG, "signInWithEmailFailure", task.getException());
                        Toast.makeText(AdminActivity.this, "Authentication Failed", Toast.LENGTH_LONG).show();
                    }

                }

            });

MainActivity.java

studentListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

                @Override
                public void onItemClick(final AdapterView<?> parent, View view, int position, long id) {

                    selectedItem = studentDetails.get(position).getName();

                    AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(MainActivity.this);

                    alertDialogBuilder.setPositiveButton("View", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            Toast.makeText(MainActivity.this, selectedItem, Toast.LENGTH_SHORT).show();
                            Intent intent = new Intent(MainActivity.this, DetailsActivity.class);
                            intent.putExtra("name", selectedItem);
                            startActivity(intent);
                        }
                    });

                    alertDialogBuilder.setNegativeButton("Delete", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            String user = null;

                            if(user.equals(intent.getStringExtra("name"))) {
                                Toast.makeText(MainActivity.this, "Authorized", Toast.LENGTH_SHORT).show();
                            } else {
                                Toast.makeText(MainActivity.this, "Not authorized", Toast.LENGTH_SHORT).show();
                            }
                        }
                    });

                    AlertDialog alertDialog = alertDialogBuilder.create();
                    alertDialog.show();
                }
            });