fix the QML demo

Author: wangwenx190

examples/qml/main.qml

@@ -12,8 +12,8 @@ Window {
     title: qsTr("Hello, world!")
     Component.onCompleted: {
         windowAgent.setup(window)
-        windowAgent.setWindowAttribute("dark-mode", true)
         window.visible = true
+        delayInitTimer.start()
     }
 
     QtObject {
@@ -32,6 +32,14 @@ Window {
         onTriggered: timeLabel.text = Qt.formatTime(new Date(), "hh:mm:ss")
     }
 
+    Timer {
+        id: delayInitTimer
+        interval: 100
+        running: false
+        repeat: false
+        onTriggered: windowAgent.setWindowAttribute("dark-mode", true)
+    }
+
     WindowAgent {
         id: windowAgent
     }