fix bug

lib/src/openim.dart

@@ -0,0 +1,9 @@
+import 'package:flutter/services.dart';
+import 'package:flutter_openim_sdk/flutter_openim_sdk.dart';
+
+class OpenIM {
+  static const MethodChannel _channel =
+      const MethodChannel('flutter_openim_sdk');
+
+  static IMManager iMManager = IMManager(_channel);
+}