Update MainFlutterWindow.swift

{{cookiecutter.out_dir}}/macos/Runner/MainFlutterWindow.swift

@@ -1,5 +1,6 @@
 import Cocoa
 import FlutterMacOS
+import window_manager
 
 class MainFlutterWindow: NSWindow {
   override func awakeFromNib() {
@@ -12,4 +13,9 @@ class MainFlutterWindow: NSWindow {
 
     super.awakeFromNib()
   }
+
+  override public func order(_ place: NSWindow.OrderingMode, relativeTo otherWin: Int) {
+    super.order(place, relativeTo: otherWin)
+    hiddenWindowAtLaunch()
+  }
 }