Can sign out of the application

lib/helpers/account_credentials_helper.dart

@@ -10,7 +10,6 @@ import 'package:shared_preferences/shared_preferences.dart';
 /// @author Pierre HUBERT
 
 class AccountCredentialsHelper {
-
   /// Checkout whether current user is signed in or not
   Future<bool> signedIn() async {
     return await get() != null;
@@ -19,21 +18,20 @@ class AccountCredentialsHelper {
   /// Set new login tokens
   Future<void> set(LoginTokens tokens) async {
     SharedPreferences prefs = await SharedPreferences.getInstance();
-    await prefs.setString("login_tokens", tokens.toString());
+    await prefs.setString(
+        "login_tokens", tokens == null ? "null" : tokens.toString());
   }
 
   /// Get current [LoginTokens]. Returns null if none or in case of failure
   Future<LoginTokens> get() async {
     try {
-
       SharedPreferences prefs = await SharedPreferences.getInstance();
       final string = prefs.getString("login_tokens");
-      if(string == null) return null;
+      if (string == null || string == "null") return null;
       return LoginTokens.fromJSON(jsonDecode(string));
-
-    } on Exception catch(e){
+    } on Exception catch (e) {
       print(e.toString());
       return null;
     }
   }
-}
+}

lib/helpers/account_helper.dart

@@ -38,4 +38,10 @@ class AccountHelper {
 
     return AuthResult.SUCCESS;
   }
+
+  /// Sign out user
+  Future<void> signOut() async {
+    await AccountCredentialsHelper().set(null);
+  }
+
 }