QWindow::fromWinId(): Return 0 when foreign window cannot be wrapped

Change window creation code in QWindow to not assert should
platform window creation fail for foreign windows.
Prototypically add check to the Windows QPA plugin.

[ChangeLog][Windows][Important Behavioral Changes] QWindow::fromWinId()
may return 0 when passing invalid window handles.

Task-number: QTBUG-41186
Change-Id: I936112607ec6e0838d36ac2a72aa88b869df5c23
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 6 insertions, 1 deletions

diff --git a/src/plugins/platforms/windows/qwindowsintegration.cpp b/src/plugins/platforms/windows/qwindowsintegration.cpp
index 6142aac92e..debe6dd131 100644
--- a/src/plugins/platforms/windows/qwindowsintegration.cpp
+++ b/src/plugins/platforms/windows/qwindowsintegration.cpp
@@ -311,7 +311,12 @@ QPlatformWindow *QWindowsIntegration::createPlatformWindow(QWindow *window) cons
     }
 
     if (window->type() == Qt::ForeignWindow) {
-        QWindowsForeignWindow *result = new QWindowsForeignWindow(window, reinterpret_cast<HWND>(window->winId()));
+        const HWND hwnd = reinterpret_cast<HWND>(window->winId());
+        if (!IsWindow(hwnd)) {
+           qWarning("Windows QPA: Invalid foreign window ID %p.");
+           return nullptr;
+        }
+        QWindowsForeignWindow *result = new QWindowsForeignWindow(window, hwnd);
         const QRect obtainedGeometry = result->geometry();
         QScreen *screen = Q_NULLPTR;
         if (const QPlatformScreen *pScreen = result->screenForGeometry(obtainedGeometry))