Add missing nullptr check in QWidget::setFocusProxy

b1802a164b added handling for a parent to
become focus proxy of a child. The respective 'else if' branch didn't
check whether setFocusProxy() was called with a nullptr argument.

This patch adds the missing nullptr check.
It also adds functionality to tst_QWidget::tabOrderComboBox() to test
the removal of a focus proxy, as well as the complete removal of an
element from the focus chain.

Change-Id: I4cb865b9ac4497fc5e2595910738fb77694f5837
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -6412,7 +6412,7 @@ void QWidget::setFocusProxy(QWidget * w)
         d->focus_prev = oldPrev;
         oldPrev->d_func()->focus_next = this;
         firstChild->d_func()->focus_prev = this;
-    } else if (w->isAncestorOf(this)) {
+    } else if (w && w->isAncestorOf(this)) {
         // If the focus proxy is a parent, 'this' has to be inserted directly after its parent in the focus chain
         // remove it from the chain and insert this into the focus chain after its parent
 

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -2144,6 +2144,7 @@ void tst_QWidget::tabOrderComboBox()
     QFETCH(const QList<int>, secondTabOrder);
     const int count = firstTabOrder.count();
     Q_ASSERT(count == secondTabOrder.count());
+    Q_ASSERT(count > 1);
 
     QWidget w;
     w.setObjectName("MainWidget");
@@ -2176,6 +2177,31 @@ void tst_QWidget::tabOrderComboBox()
 
     COMPARE(secondTabOrder);
 
+    // Remove the focus proxy of the first combobox's line edit.
+    QComboBox *box = boxes.at(0);
+    QLineEdit *lineEdit = box->lineEdit();
+    QWidgetPrivate *lePriv = QWidgetPrivate::get(lineEdit);
+    const QWidget *prev = lePriv->focus_prev;
+    const QWidget *next = lePriv->focus_next;
+    const QWidget *proxy = lePriv->extra->focus_proxy;
+    QCOMPARE(proxy, box);
+    lineEdit->setFocusProxy(nullptr);
+    QCOMPARE(lePriv->extra->focus_proxy, nullptr);
+    QCOMPARE(lePriv->focus_prev, prev);
+    QCOMPARE(lePriv->focus_next, next);
+
+    // Remove first item and check chain consistency
+    boxes.removeFirst();
+    delete box;
+
+    // Create new list with 0 removed and other indexes updated
+    QList<int> thirdTabOrder(secondTabOrder);
+    thirdTabOrder.removeIf([](int i){ return i == 0; });
+    for (int &i : thirdTabOrder)
+        --i;
+
+    COMPARE(thirdTabOrder);
+
 #undef COMPARE
 }