Click the dock icon to restore after closing the window

Change the file macos/Runner/AppDelegate.swift as follows:

import Cocoa
import FlutterMacOS

@NSApplicationMain
class AppDelegate: FlutterAppDelegate {
    override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
        return false
    }

+    override func applicationShouldHandleReopen(_ sender: NSApplication, hasVisibleWindows flag: Bool) -> Bool {
+        if !flag {
+            for window in NSApp.windows {
+                if !window.isVisible {
+                    window.setIsVisible(true)
+                }
+                window.makeKeyAndOrderFront(self)
+                NSApp.activate(ignoringOtherApps: true)
+            }
+        }
+        return true
+    }
}