input: don't crash if the internal handle is nullptr (wheelEvent)

Apply the same fix as 4d9a0d23 to wheelEvent

(cherry picked from commit e0b554e67b)

src/input.cpp

@@ -1238,6 +1238,10 @@ public:
             return false;
         }
         QWindow *internal = static_cast<InternalWindow *>(input()->pointer()->focus())->handle();
+        if (!internal) {
+            // the handle can be nullptr if the tooltip gets closed while focus updates are blocked
+            return false;
+        }
         const QPointF localPos = event->globalPosition() - internal->position();
         QWheelEvent wheelEvent(localPos, event->globalPosition(), QPoint(),
                                event->angleDelta() * -1,