[Auth] Main queue dispatch for 'UIApplication / open(_:options:completionHandler:)'

FirebaseAuth/Sources/Swift/MultiFactor/TOTP/TOTPSecret.swift

@@ -56,24 +56,28 @@ import Foundation
     /// [here](https://developer.apple.com/documentation/authenticationservices/securing_logins_with_icloud_keychain_verification_codes)
     @objc(openInOTPAppWithQRCodeURL:)
     open func openInOTPApp(withQRCodeURL qrCodeURL: String) {
-      if GULAppEnvironmentUtil.isAppExtension() {
-        // iOS App extensions should not call [UIApplication sharedApplication], even if
-        // UIApplication responds to it.
-        return
-      }
+      DispatchQueue.main.async {
+        if GULAppEnvironmentUtil.isAppExtension() {
+          // iOS App extensions should not call [UIApplication sharedApplication], even if
+          // UIApplication responds to it.
+          return
+        }
 
-      // Using reflection here to avoid build errors in extensions.
-      let sel = NSSelectorFromString("sharedApplication")
-      guard UIApplication.responds(to: sel),
-            let rawApplication = UIApplication.perform(sel),
-            let application = rawApplication.takeUnretainedValue() as? UIApplication else {
-        return
-      }
-      if let url = URL(string: qrCodeURL), application.canOpenURL(url) {
-        application.open(url, options: [:], completionHandler: nil)
-      } else {
-        AuthLog.logError(code: "I-AUT000019",
-                         message: "URL: \(qrCodeURL) cannot be opened")
+        // Using reflection here to avoid build errors in extensions.
+        // The `UIApplication.sharedApplication` requires main actor
+        // synchronization.
+        let sel = NSSelectorFromString("sharedApplication")
+        guard UIApplication.responds(to: sel),
+              let rawApplication = UIApplication.perform(sel),
+              let application = rawApplication.takeUnretainedValue() as? UIApplication else {
+          return
+        }
+        if let url = URL(string: qrCodeURL), application.canOpenURL(url) {
+          application.open(url, options: [:], completionHandler: nil)
+        } else {
+          AuthLog.logError(code: "I-AUT000019",
+                           message: "URL: \(qrCodeURL) cannot be opened")
+        }
       }
     }