diff -Naur qt-everywhere-src-5.12.4-orig/qtbase/src/widgets/dialogs/qmessagebox.cpp qt-everywhere-src-5.12.4/qtbase/src/widgets/dialogs/qmessagebox.cpp
--- qt-everywhere-src-5.12.4-orig/qtbase/src/widgets/dialogs/qmessagebox.cpp	2019-06-12 23:59:14.000000000 +0300
+++ qt-everywhere-src-5.12.4/qtbase/src/widgets/dialogs/qmessagebox.cpp	2019-06-15 15:58:57.794713900 +0300
@@ -77,9 +77,11 @@
 #if defined(Q_OS_WIN) && !defined(Q_OS_WINRT)
 HMENU qt_getWindowsSystemMenu(const QWidget *w)
 {
-    if (QWindow *window = QApplicationPrivate::windowForWidget(w))
-        if (void *handle = QGuiApplication::platformNativeInterface()->nativeResourceForWindow("handle", window))
-            return GetSystemMenu(reinterpret_cast<HWND>(handle), false);
+    if (QGuiApplication::platformNativeInterface()) {
+        if (QWindow *window = QApplicationPrivate::windowForWidget(w))
+            if (void *handle = QGuiApplication::platformNativeInterface()->nativeResourceForWindow("handle", window))
+                return GetSystemMenu(reinterpret_cast<HWND>(handle), false);
+    }
     return 0;
 }
 #endif
diff -Naur qt-everywhere-src-5.12.4-orig/qtbase/src/widgets/dialogs/qwizard_win.cpp qt-everywhere-src-5.12.4/qtbase/src/widgets/dialogs/qwizard_win.cpp
--- qt-everywhere-src-5.12.4-orig/qtbase/src/widgets/dialogs/qwizard_win.cpp	2019-06-12 23:59:14.000000000 +0300
+++ qt-everywhere-src-5.12.4/qtbase/src/widgets/dialogs/qwizard_win.cpp	2019-06-15 15:58:57.810314000 +0300
@@ -176,10 +176,11 @@
         // The dynamic property takes effect when creating the platform window.
         window->setProperty("_q_windowsCustomMargins", customMarginsV);
         // If a platform window exists, change via native interface.
-        if (QPlatformWindow *platformWindow = window->handle()) {
-            QGuiApplication::platformNativeInterface()->
-                setWindowProperty(platformWindow, QStringLiteral("WindowsCustomMargins"),
-                                  customMarginsV);
+        if (QGuiApplication::platformNativeInterface()) {
+            if (QPlatformWindow *platformWindow = window->handle())
+                QGuiApplication::platformNativeInterface()->
+                    setWindowProperty(platformWindow, QStringLiteral("WindowsCustomMargins"),
+                                      customMarginsV);
         }
     }
 }
@@ -558,6 +559,8 @@
 // wizard is a child window.
 HDC QVistaHelper::backingStoreDC(const QWidget *wizard, QPoint *offset)
 {
+    if (!QGuiApplication::platformNativeInterface())
+        return static_cast<HDC>(0);
     HDC hdc = static_cast<HDC>(QGuiApplication::platformNativeInterface()->nativeResourceForBackingStore(QByteArrayLiteral("getDC"), wizard->backingStore()));
     *offset = QPoint(0, 0);
     if (!wizard->windowHandle())
@@ -572,7 +575,7 @@
     // Do not use winId() as this enforces native children of the parent
     // widget when called before show() as happens when calling setWizardStyle().
     if (QWindow *window = wizard->windowHandle())
-        if (window->handle())
+        if (window->handle() && QGuiApplication::platformNativeInterface())
             if (void *vHwnd = QGuiApplication::platformNativeInterface()->nativeResourceForWindow(QByteArrayLiteral("handle"), window))
                 return static_cast<HWND>(vHwnd);
     qWarning().nospace() << "Failed to obtain HWND for wizard.";