🐛 Revert accidental breaking change (#142)

The previous release accidentally broke compatibility with Flutter v2, but made it compatible with Flutter v3. This commit reverts that so that the 0.4.x release line stays compatible with Flutter v2.

Author: devmvk

lib/flutter_web_auth.dart

@@ -36,8 +36,8 @@ class FlutterWebAuth {
       throw ArgumentError.value(callbackUrlScheme, 'callbackUrlScheme', 'must be a valid URL scheme');
     }
 
-    WidgetsBinding.instance.removeObserver(_resumedObserver); // safety measure so we never add this observer twice
-    WidgetsBinding.instance.addObserver(_resumedObserver);
+    WidgetsBinding.instance?.removeObserver(_resumedObserver); // safety measure so we never add this observer twice
+    WidgetsBinding.instance?.addObserver(_resumedObserver);
     return await _channel.invokeMethod('authenticate', <String, dynamic>{
       'url': url,
       'callbackUrlScheme': callbackUrlScheme,
@@ -50,6 +50,6 @@ class FlutterWebAuth {
   /// terminate all `authenticate` calls with an error.
   static Future<void> _cleanUpDanglingCalls() async {
     await _channel.invokeMethod('cleanUpDanglingCalls');
-    WidgetsBinding.instance.removeObserver(_resumedObserver);
+    WidgetsBinding.instance?.removeObserver(_resumedObserver);
   }
 }