Port a few more changes from iOS to macOS

flutter_web_auth_2/macos/Classes/FlutterWebAuth2Plugin.swift

@@ -5,20 +5,22 @@ import FlutterMacOS
 @available(OSX 10.15, *)
 public class FlutterWebAuth2Plugin: NSObject, FlutterPlugin, ASWebAuthenticationPresentationContextProviding {
     public static func register(with registrar: FlutterPluginRegistrar) {
-        let channel = FlutterMethodChannel(name: "flutter_web_auth_2", binaryMessenger: registrar.messenger)
+        let channel = FlutterMethodChannel(name: "flutter_web_auth_2", binaryMessenger: registrar.messenger())
         let instance = FlutterWebAuth2Plugin()
         registrar.addMethodCallDelegate(instance, channel: channel)
     }
 
     public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
-        if call.method == "authenticate" {
-            let url = URL(string: (call.arguments as! Dictionary<String, AnyObject>)["url"] as! String)!
-            let callbackURLScheme = (call.arguments as! Dictionary<String, AnyObject>)["callbackUrlScheme"] as! String
-            let options = (call.arguments as! Dictionary<String, AnyObject>)["options"] as? Dictionary<String, AnyObject>
-
-            var keepMe: Any? = nil
+        if call.method == "authenticate",
+           let arguments = call.arguments as? [String: AnyObject],
+           let urlString = arguments["url"] as? String,
+           let url = URL(string: urlString),
+           let callbackURLScheme = arguments["callbackUrlScheme"] as? String,
+           let options = arguments["options"] as? [String: AnyObject]
+        {
+            var sessionToKeepAlive: Any? // if we do not keep the session alive, it will get closed immediately while showing the dialog
             let completionHandler = { (url: URL?, err: Error?) in
-                keepMe = nil
+                sessionToKeepAlive = nil
 
                 if let err = err {
                     if case ASWebAuthenticationSessionError.canceledLogin = err {
@@ -30,17 +32,25 @@ public class FlutterWebAuth2Plugin: NSObject, FlutterPlugin, ASWebAuthentication
                     return
                 }
 
-                result(url!.absoluteString)
+                guard let url = url else {
+                    result(FlutterError(code: "EUNKNOWN", message: "URL was null, but no error provided.", details: nil))
+                    return
+                }
+
+                result(url.absoluteString)
             }
 
             let session = ASWebAuthenticationSession(url: url, callbackURLScheme: callbackURLScheme, completionHandler: completionHandler)
-            if let preferEphemeral = options?["preferEphemeral"] as? Bool {
+            if let preferEphemeral = options["preferEphemeral"] as? Bool {
                 session.prefersEphemeralWebBrowserSession = preferEphemeral
             }
             session.presentationContextProvider = self
 
             session.start()
-            keepMe = session
+            sessionToKeepAlive = session
+        } else if call.method == "cleanUpDanglingCalls" {
+            // we do not keep track of old callbacks on macOS, so nothing to do here
+            result(nil)
         } else {
             result(FlutterMethodNotImplemented)
         }