Use a key for main controller

lib/ui/routes/main_route/main_route.dart

@@ -6,13 +6,21 @@ import 'package:flutter/material.dart';
 ///
 /// @author Pierre Hubert
 
+/// Main controller key statically shared across application
+///
+/// Do not use it directly to avoid context leak, instead use the access method
+/// [MainController.of]
+final mainControllerKey = GlobalKey<MainController>();
+
 mixin MainRoute implements StatefulWidget {}
 
 /// Public interface of home controller
 mixin MainController implements State<MainRoute> {
   /// Get current instance of Home controller
-  static MainController of(BuildContext context) =>
-      context.findAncestorStateOfType<MainController>();
+  static MainController of(BuildContext context) {
+    assert(context != null); // A future implementation might need context again
+    return mainControllerKey.currentState;
+  }
 
   /// Open user page
   void openUserPage(int userID);

lib/ui/routes/main_route/smartphone_route.dart

@@ -27,6 +27,8 @@ import '../login_route.dart';
 /// @author Pierre HUBERT
 
 class SmartphoneMainRoute extends StatefulWidget implements MainRoute {
+  const SmartphoneMainRoute({Key key}) : super(key: key);
+
   @override
   State<StatefulWidget> createState() => _MainRouteState();
 }

lib/ui/routes/main_route/tablet_route.dart

@@ -11,6 +11,8 @@ import 'package:flutter/material.dart';
 /// @author Pierre Hubert
 
 class TabletRoute extends StatefulWidget implements MainRoute {
+  const TabletRoute({Key key}) : super(key: key);
+
   @override
   _TabletRouteState createState() => _TabletRouteState();
 }