App.cs: Fix UserAgentFallbackAsync

Author: softworkz

src/ElectronNET.API/API/App.cs

@@ -1298,7 +1298,20 @@ public Task<string> UserAgentFallbackAsync
         {
             get
             {
-                return this.GetPropertyAsync<string>();
+                return Task.Run<string>(() =>
+                {
+                    var taskCompletionSource = new TaskCompletionSource<string>();
+
+                    BridgeConnector.Socket.On("appGetUserAgentFallbackCompleted", (result) =>
+                    {
+                        BridgeConnector.Socket.Off("appGetUserAgentFallbackCompleted");
+                        taskCompletionSource.SetResult((string)result);
+                    });
+
+                    BridgeConnector.Socket.Emit("appGetUserAgentFallback");
+
+                    return taskCompletionSource.Task;
+                });
             }
         }