Fix QWasmWindowStack::topWindow when the stack is empty

The method should by contract return nullptr when the stack is empty,
but it crashes in this case.

Also, unit-test the case.

Pick-to: 6.4
Change-Id: If64b71e761efd9a5cd5af407cd68cba7f8dbc8e2
Reviewed-by: Morten Johan Sørvig <[email protected]>

2 files changed, 28 insertions, 1 deletions

diff --git a/src/plugins/platforms/wasm/qwasmwindowstack.cpp b/src/plugins/platforms/wasm/qwasmwindowstack.cpp
index cc1a3cad6af..c73ecde9fbe 100644
--- a/src/plugins/platforms/wasm/qwasmwindowstack.cpp
+++ b/src/plugins/platforms/wasm/qwasmwindowstack.cpp
@@ -111,7 +111,7 @@ QWasmWindow *QWasmWasmWindowStack::rootWindow() const
 
 QWasmWindow *QWasmWasmWindowStack::topWindow() const
 {
-    return m_windowStack.last();
+    return m_windowStack.empty() ? nullptr : m_windowStack.last();
 }
 
 QWasmWasmWindowStack::StorageType::iterator QWasmWasmWindowStack::regularWindowsBegin()
diff --git a/tests/auto/wasm/tst_qwasmwindowstack.cpp b/tests/auto/wasm/tst_qwasmwindowstack.cpp
index f978679a15e..62b47bca6db 100644
--- a/tests/auto/wasm/tst_qwasmwindowstack.cpp
+++ b/tests/auto/wasm/tst_qwasmwindowstack.cpp
@@ -36,6 +36,7 @@ private slots:
     void lowering();
     void removing();
     void removingTheRoot();
+    void clearing();
 
 private:
     void onTopWindowChanged()
@@ -241,5 +242,31 @@ void tst_QWasmWindowStack::removingTheRoot()
                        getWindowsFrontToBack(&stack).begin()));
 }
 
+void tst_QWasmWindowStack::clearing()
+{
+    QWasmWasmWindowStack stack(m_mockCallback);
+
+    stack.pushWindow(&m_root);
+    stack.pushWindow(&m_window1);
+    // Window order: 1 R
+
+    clearCallbackCounter();
+
+    QCOMPARE(&m_window1, stack.topWindow());
+
+    m_onTopLevelChangedAction = [this, &stack]() { QVERIFY(stack.topWindow() == &m_root); };
+    stack.removeWindow(&m_window1);
+    // Window order: R
+    verifyTopWindowChangedCalled();
+    QCOMPARE(&m_root, stack.topWindow());
+
+    m_onTopLevelChangedAction = [&stack]() { QVERIFY(stack.topWindow() == nullptr); };
+    stack.removeWindow(&m_root);
+    // Window order: <empty>
+    verifyTopWindowChangedCalled();
+    QCOMPARE(nullptr, stack.topWindow());
+    QCOMPARE(0u, stack.size());
+}
+
 QTEST_MAIN(tst_QWasmWindowStack)
 #include "tst_qwasmwindowstack.moc"