Fix for crash while closing document and moving the stylus

Now it is checked if the object who should receive the event is null or not.

Restored the usage of retval.

BUG:342782

krita/ui/input/kis_input_manager.cpp

@@ -861,21 +861,23 @@ bool KisInputManager::eventFilter(QObject* object, QEvent* event)
 
 bool KisInputManager::Private::handleKisTabletEvent(QObject *object, KisTabletEvent *tevent)
 {
+    if(object == 0) return false;
+
     bool retval = false;
 
     QTabletEvent qte = tevent->toQTabletEvent();
     qte.ignore();
-    q->eventFilter(object, &qte);
+    retval = q->eventFilter(object, &qte);
     tevent->setAccepted(qte.isAccepted());
 
     if (!retval && !qte.isAccepted()) {
         QMouseEvent qme = tevent->toQMouseEvent();
         qme.ignore();
-        q->eventFilter(object, &qme);
+        retval = q->eventFilter(object, &qme);
         tevent->setAccepted(qme.isAccepted());
     }
 
-    return tevent->isAccepted();
+    return retval;
 }
 
 void KisInputManager::slotCompressedMoveEvent()