fix: 一个进程多次实例化QApplication程序崩溃。

QApplication在析构时会引起DPlatformIntegration的析构，DPlatformIntegration析构会释放其静态成员变量m_xsettings。
DGuiApplicationHelper为全局变量，只会init一次，创建DPlatformTheme，
DPlatformIntegration::buildNativeSettings在new DNativeSettings时会拷贝DPlatformIntegration的m_xsettings变量，
保存在其成员变量m_settings里。
当QApplication第二次构造时，重新获取调色板，会通过DGuiApplicationHelper访问应用主题，
继而会访问到DNativeSettings指向的m_settings成员变量，
此时m_settings指向的内存已经在上一次QApplication析构时释放掉了，成为了野指针，导致程序崩溃。

在QApplication析构之后再将DGuiApplicationHelper析构，
需要时再重新构造DGuiApplicationHelper，再次init，创建DPlatformTheme，
可保证DNativeSettings跟QApplication实例化次数无关，每次都重新构造。

Log:
Change-Id: I1aaba6397124afb3b95cc46c82db0efb2999a9aa

src/kernel/dguiapplicationhelper.cpp

@@ -427,14 +427,16 @@ class _DGuiApplicationHelper
 {
 public:
     _DGuiApplicationHelper()
-        : helper(creator())
+        : helper(nullptr)
     {
-        helper->initialize();
     }
 
     ~_DGuiApplicationHelper()
     {
-        delete helper;
+        if (helper) {
+            helper->deleteLater();
+            helper = nullptr;
+        }
     }
 
     static DGuiApplicationHelper *defaultCreator()
@@ -497,9 +499,8 @@ void DGuiApplicationHelper::registerInstanceCreator(DGuiApplicationHelper::Helpe
     _DGuiApplicationHelper::creator = creator;
 
     if (_globalHelper.exists() && _globalHelper->helper) {
-        delete _globalHelper->helper;
-        _globalHelper->helper = creator();
-        _globalHelper->helper->initialize();
+        _globalHelper->helper->deleteLater();
+        _globalHelper->helper = nullptr;
     }
 }
 
@@ -515,6 +516,19 @@ inline static int adjustColorValue(int base, qint8 increment, int max = 255)
  */
 DGuiApplicationHelper *DGuiApplicationHelper::instance()
 {
+    // 单例模式，延迟创建DGuiApplicationHelper
+    if (nullptr == _globalHelper->helper) {
+        _globalHelper->helper = _DGuiApplicationHelper::creator();
+        _globalHelper->helper->initialize();
+        // 当QApplication析构时，同时析构DGuiApplicationHelper
+        QObject::connect(qApp, &QObject::destroyed, [](){
+            if (_globalHelper->helper) {
+                _globalHelper->helper->deleteLater();
+                _globalHelper->helper = nullptr;
+            }
+        });
+    }
+
     return _globalHelper->helper;
 }