Pages on Navigator stack rebuild when a new page is pushed in Flutter

All you need to do is move this row

final _formKey = new GlobalKey<FormState>();

from build method to state class declaration (i.e. outside from build). Key must be created once when class is created. In your case the key is re-created each build action.

I solved the problem by simply changing the class like this :

import 'package:flutter/material.dart';

class UserLoader extends StatefulWidget {
  @override
  _UserLoaderState createState() => new _UserLoaderState();
}

class _UserLoaderState extends State<UserLoader> {
  Widget _form; // Save the form

  @override
  Widget build(BuildContext context) {
    if (_form == null) { // Create the form if it does not exist
      _form = _createForm(context); // Build the form
    }
    return _form; // Show the form in the application
  }

  Widget _createForm(BuildContext context) {
    // This is the exact content of the build method in the question

    final _formKey = new GlobalKey<FormState>();
    final _emailController = new TextEditingController();

    return new Scaffold(
        appBar: new AppBar(
          title: new Text("Informations"),
          actions: <Widget>[
            new IconButton(
                icon: const Icon(Icons.save),
                onPressed: () {
                  // unrelated stuff happens here
                })
          ],
        ),
        body: new Center(
          child: new SingleChildScrollView(
              child: new Form(
                  key: _formKey,
                  child: new Column(children: <Widget>[
                    new ListTile(
                      leading: const Icon(Icons.email),
                      title: new TextFormField(
                        decoration: new InputDecoration(
                          hintText: "Email",
                        ),
                        keyboardType: TextInputType.emailAddress,
                        controller: _emailController,
                        validator: _validateEmail,
                      ),
                    ),
                  ]))),
        ));
    }
  }
}

Hope this may help someone else someday.