Fixed crash taking null central widget

When no central widget has been set, calling takeCentralWidget should
just return a null pointer instead of crashing.

[ChangeLog][QtWidgets][QMainWindow] Fixed crash using takeCentralWidget when
the central widget was not set.

Task-number: QTBUG-56628
Change-Id: I240ccf4caa41d2716a78851571fbfbf444a4922e
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

2 files changed, 8 insertions, 2 deletions

diff --git a/src/widgets/widgets/qmainwindow.cpp b/src/widgets/widgets/qmainwindow.cpp
index 50ba7f97ec..e454e3e991 100644
--- a/src/widgets/widgets/qmainwindow.cpp
+++ b/src/widgets/widgets/qmainwindow.cpp
@@ -664,8 +664,10 @@ QWidget *QMainWindow::takeCentralWidget()
 {
     Q_D(QMainWindow);
     QWidget *oldcentralwidget = d->layout->centralWidget();
-    oldcentralwidget->setParent(0);
-    d->layout->setCentralWidget(0);
+    if (oldcentralwidget) {
+        oldcentralwidget->setParent(0);
+        d->layout->setCentralWidget(0);
+    }
     return oldcentralwidget;
 }
 
diff --git a/tests/auto/widgets/widgets/qmainwindow/tst_qmainwindow.cpp b/tests/auto/widgets/widgets/qmainwindow/tst_qmainwindow.cpp
index 6282028746..ece011d145 100644
--- a/tests/auto/widgets/widgets/qmainwindow/tst_qmainwindow.cpp
+++ b/tests/auto/widgets/widgets/qmainwindow/tst_qmainwindow.cpp
@@ -864,6 +864,10 @@ void tst_QMainWindow::takeCentralWidget() {
 
     QVERIFY(!mw.centralWidget());
 
+    // verify that we don't crash when trying to take a non-set
+    // central widget but just return a null pointer instead
+    QVERIFY(!mw.takeCentralWidget());
+
     mw.setCentralWidget(w1);
 
     QWidget *oldCentralWidget = mw.takeCentralWidget();