wasm: Fix nullptr access in QWasmScreen::allwindowsHEADdev

If the QWindow::handle() has not been created yet we will add
a zero page pointer to the allwindows list.
One possible symptom is that emscripten::val complains
in context2d() that it is accessed by the wrong thread.
what happens is that the this pointer points to the first
page.
Seen in the documentviewer example

Change-Id: I2b08176ebdd7c35d5105194f7fd9f4f6d45b77e5
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

1 files changed, 8 insertions, 4 deletions

diff --git a/src/plugins/platforms/wasm/qwasmscreen.cpp b/src/plugins/platforms/wasm/qwasmscreen.cpp
index ddf8140c48..0490b2bfe0 100644
--- a/src/plugins/platforms/wasm/qwasmscreen.cpp
+++ b/src/plugins/platforms/wasm/qwasmscreen.cpp
@@ -361,10 +361,14 @@ QList<QWasmWindow *> QWasmScreen::allWindows()
 {
     QList<QWasmWindow *> windows;
     for (auto *child : childStack()) {
-        QWindowList list = child->window()->findChildren<QWindow *>(Qt::FindChildrenRecursively);
-        std::transform(
-                list.begin(), list.end(), std::back_inserter(windows),
-                [](const QWindow *window) { return static_cast<QWasmWindow *>(window->handle()); });
+        const QWindowList list = child->window()->findChildren<QWindow *>(Qt::FindChildrenRecursively);
+        for (auto child : list) {
+            auto handle = child->handle();
+            if (handle) {
+                auto wnd = static_cast<QWasmWindow *>(handle);
+                windows.push_back(wnd);
+            }
+        }
         windows.push_back(child);
     }
     return windows;