Fix server port bug and int parse bug

[memoriasIT]

This PR will fix two bugs.

1. In the example there is an error in parsing the height:

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: FormatException: Invalid radix-10 number (at character 1)
136.0
^

#0      int._handleFormatError (dart:core-patch/integers_patch.dart:127:5)
#1      int._parseRadix (dart:core-patch/integers_patch.dart:138:16)
#2      int._parse (dart:core-patch/integers_patch.dart:99:12)
#3      int.parse (dart:core-patch/integers_patch.dart:61:12)
#4      _MainPageState.initState.<anonymous closure>.<anonymous closure> (package:webview_flutter_plus_example/main.dart:44:32)
#5      Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:902:45)
#6      Future._propagateToListeners (dart:async/future_impl.dart:931:13)

2. Server port being closed already (issue HttpException: HttpServer is not bound to a socket #57)

flutter: HttpException: HttpServer is not bound to a socket
flutter: 
#0      _HttpServer.port (dart:_http/http_impl.dart:3324:17)
#1      LocalhostServer.port (package:webview_flutter_plus/src/localhost_server.dart:26:29)
#2      LocalhostServer.close (package:webview_flutter_plus/src/localhost_server.dart:105:50)