Add a placeholder implementation for non-mobile platforms (#688)

Author: swift-kim

packages/flutter_reactive_ble/lib/src/reactive_ble.dart

@@ -104,9 +104,12 @@ class FlutterReactiveBle {
         print,
       );
 
-      ReactiveBlePlatform.instance = const ReactiveBleMobilePlatformFactory().create(
-        logger: _debugLogger,
-      );
+      if (Platform.isAndroid || Platform.isIOS) {
+        ReactiveBlePlatform.instance =
+            const ReactiveBleMobilePlatformFactory().create(
+          logger: _debugLogger,
+        );
+      }
 
       _blePlatform = ReactiveBlePlatform.instance;
 

packages/reactive_ble_platform_interface/lib/src/reactive_ble_platform_interface.dart

@@ -13,7 +13,7 @@ abstract class ReactiveBlePlatform extends PlatformInterface {
   ReactiveBlePlatform() : super(token: _token);
   static final Object _token = Object();
 
-  static late ReactiveBlePlatform _instance;
+  static ReactiveBlePlatform _instance = _PlaceholderImplementation();
 
   static ReactiveBlePlatform get instance => _instance;
 
@@ -186,3 +186,5 @@ abstract class ReactiveBlePlatform extends PlatformInterface {
         'requestConnectionPriority has not been implemented.');
   }
 }
+
+class _PlaceholderImplementation extends ReactiveBlePlatform {}