build ios warning: incompatible pointer to integer conversion assigning to 'BOOL' (aka 'signed char') from 'id _Nullable'

` /Users/furun/.pub-cache/hosted/pub.flutter-io.cn/flutter_webview_plugin-0.3.4/ios/Classes/FlutterWebviewPlugin.m:83:22: warning: incompatible pointer to integer conversion assigning to 'BOOL' (aka 'signed char') from 'id _Nullable'
[-Wint-conversion]
_enableAppScheme = call.arguments[@"enableAppScheme"];
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/furun/.pub-cache/hosted/pub.flutter-io.cn/flutter_webview_plugin-0.3.4/ios/Classes/FlutterWebviewPlugin.m:252:24: warning: comparison of distinct pointer types ('NSString *' and 'NSNull * _Nonnull') [-Wcompare-distinct-pointer-types]
if (_invalidUrlRegex != [NSNull null] && urlString != nil) {
~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~
/Users/furun/.pub-cache/hosted/pub.flutter-io.cn/flutter_webview_plugin-0.3.4/ios/Classes/FlutterWebviewPlugin.m:318:94: warning: values of type 'NSInteger' should not be used as format arguments; add an explicit cast to 'long' instead
[-Wformat]
[channel invokeMethod:@"onError" arguments:@{@"code": [NSString stringWithFormat:@"%ld", error.code], @"error": error.localizedDescription}];
~~~ ^~~~~~~~~~
%ld (long)
`

Start in ios/Classes/FlutterWebviewPlugin.m at the warnings around lines 83, 252, and 318. Build the iOS example or plugin to reproduce the compiler diagnostics, then verify that the three incompatible conversion, pointer comparison, and format warnings are resolved without introducing new warnings.

Written by the indexing model from the issue text.