diff -urN qt-everywhere-opensource-src-5.5.0.orig/qtbase/src/widgets/dialogs/qmessagebox.cpp qt-everywhere-opensource-src-5.5.0/qtbase/src/widgets/dialogs/qmessagebox.cpp
--- qt-everywhere-opensource-src-5.5.0.orig/qtbase/src/widgets/dialogs/qmessagebox.cpp	2015-08-26 14:28:59.145124600 +0100
+++ qt-everywhere-opensource-src-5.5.0/qtbase/src/widgets/dialogs/qmessagebox.cpp	2015-08-26 15:46:50.092516900 +0100
@@ -68,9 +68,11 @@
 #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 -urN qt-everywhere-opensource-src-5.5.0.orig/qtbase/src/widgets/dialogs/qwizard_win.cpp qt-everywhere-opensource-src-5.5.0/qtbase/src/widgets/dialogs/qwizard_win.cpp
--- qt-everywhere-opensource-src-5.5.0.orig/qtbase/src/widgets/dialogs/qwizard_win.cpp	2015-08-26 14:28:59.139624600 +0100
+++ qt-everywhere-opensource-src-5.5.0/qtbase/src/widgets/dialogs/qwizard_win.cpp	2015-08-26 15:46:50.084516900 +0100
@@ -288,10 +288,11 @@
         // 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);
         }
     }
 }
@@ -685,6 +686,8 @@
 // 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())
@@ -699,7 +702,7 @@
     // 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.";
diff -urN qt-everywhere-opensource-src-5.5.0.orig/qtbase/src/widgets/kernel/qapplication_p.h qt-everywhere-opensource-src-5.5.0/qtbase/src/widgets/kernel/qapplication_p.h
--- qt-everywhere-opensource-src-5.5.0.orig/qtbase/src/widgets/kernel/qapplication_p.h	2015-08-26 14:28:59.349124600 +0100
+++ qt-everywhere-opensource-src-5.5.0/qtbase/src/widgets/kernel/qapplication_p.h	2015-08-26 15:46:50.098516900 +0100
@@ -254,10 +254,12 @@
 #ifdef Q_OS_WIN
     static HWND getHWNDForWidget(const QWidget *widget)
     {
-        if (QWindow *window = windowForWidget(widget))
-            if (window->handle())
-                return static_cast<HWND> (QGuiApplication::platformNativeInterface()->
-                                          nativeResourceForWindow(QByteArrayLiteral("handle"), window));
+        if (QGuiApplication::platformNativeInterface()) {
+            if (QWindow *window = windowForWidget(widget))
+                if (window->handle())
+                    return static_cast<HWND> (QGuiApplication::platformNativeInterface()->
+                                              nativeResourceForWindow(QByteArrayLiteral("handle"), window));
+        }
         return 0;
     }
 #endif