Better use of async functions for API

- Use local_preferences for storing data

lib/main.dart

@@ -2,11 +2,13 @@ import 'package:flutter/material.dart';
 
 import 'settings.dart';
 import 'api.dart';
+import 'preferences.dart';
 
 void main() => runApp(MyApp());
 
 class MyApp extends StatelessWidget {
   // This widget is the root of your application.
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
@@ -21,7 +23,7 @@ class MyApp extends StatelessWidget {
         // or simply save your changes to "hot reload" in a Flutter IDE).
         // Notice that the counter didn't reset back to zero; the application
         // is not restarted.
-        primarySwatch: Colors.blue,
+        primarySwatch: Colors.lightGreen,
       ),
       home: MyHomePage(title: 'InvenTree'),
     );