Fixed crashing unit tests from userAgent update (#71)

Author: migs647

Sources/Segment/Plugins/Platforms/Vendors/AppleUtils.swift

@@ -66,9 +66,15 @@ internal class iOSVendorSystem: VendorSystem {
     override var userAgent: String? {
         #if !os(tvOS)
         var userAgent: String?
-        DispatchQueue.main.sync {
-          userAgent = WKWebView().value(forKey: "userAgent") as? String
+        
+        if Thread.isMainThread {
+            userAgent = WKWebView().value(forKey: "userAgent") as? String
+        } else {
+            DispatchQueue.main.sync {
+              userAgent = WKWebView().value(forKey: "userAgent") as? String
+            }
         }
+
         return userAgent
         #else
         // webkit isn't on tvos
@@ -233,9 +239,14 @@ internal class MacOSVendorSystem: VendorSystem {
 
     override var userAgent: String? {
         var userAgent: String?
-        DispatchQueue.main.sync {
-          userAgent = WKWebView().value(forKey: "userAgent") as? String
+        if Thread.isMainThread {
+            userAgent = WKWebView().value(forKey: "userAgent") as? String
+        } else {
+            DispatchQueue.main.sync {
+              userAgent = WKWebView().value(forKey: "userAgent") as? String
+            }
         }
+        
         return userAgent
     }