title: Variable values become null after setted up in onStart
body: <p>I've tried to set some variable with <code>onStart()</code> method, but their value become null when it come to <code>onCreate()</code> method.</p>
  
  <p>This is <code>onStart()</code> method:</p>
  
  <pre><code>@Override
  protected void onStart() {
      super.onStart();
  
      Query emailQuery = mDatabase.orderByChild("emailUser").equalTo(emailUser);
      emailQuery.addValueEventListener(new ValueEventListener() {
          @Override
          public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
              for(DataSnapshot singleSnapshot : dataSnapshot.getChildren()){
                  fSaldo = (String) singleSnapshot.child("saldoUser").getValue();
                  fHeaderName = (String) singleSnapshot.child("username").getValue();
                  fHeaderMail = (String) singleSnapshot.child("emailUser").getValue();
                  phone = (String) singleSnapshot.child("phone").getValue();
  
                  Log.d("Cozmo", "Saldo: " + fSaldo); //output: not_null
              }
          }
  
          @Override
          public void onCancelled(@NonNull DatabaseError databaseError) {
  
          }
      });
  }
  </code></pre>
  
  <p>and this is <code>onCreate()</code> method:</p>
  
  <pre><code>@Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
  
      FirebaseAuth fAuth = FirebaseAuth.getInstance();
      FirebaseUser user = fAuth.getCurrentUser();
      emailUser = user.getEmail();
  
      mDatabase = FirebaseDatabase.getInstance().getReference("Users");
  
      Toolbar toolbar = findViewById(R.id.toolbar);
      setSupportActionBar(toolbar);
  
      Log.d("Cozmo", "Saldo: " + fSaldo); //output is null
  }
  </code></pre>
  
  <p>This is my full MainActivity.class:</p>
  
  <pre><code>public class MainActivity extends AppCompatActivity
      implements NavigationView.OnNavigationItemSelectedListener {
  
      private String fSaldo, fHeaderName, fHeaderMail, phone;
      private String emailUser;
      private DatabaseReference mDatabase;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {...}
  
      @Override
      protected void onStart() {...}
  }
  </code></pre>
  
  <p>I dont understand these variable become null again, and it become null after line of <code>onDataChange()</code> method passed. Are there any solutions?</p>