Fix a bug that impede to set the trackbar pos using the Qt control panel

Conflicts:
	modules/highgui/src/window_QT.cpp

modules/highgui/src/window_QT.cpp

@@ -418,12 +418,14 @@ static CvBar* icvFindBarByName(QBoxLayout* layout, QString name_bar, typeBar typ
 static CvTrackbar* icvFindTrackBarByName(const char* name_trackbar, const char* name_window, QBoxLayout* layout = NULL)
 {
     QString nameQt(name_trackbar);
-    if ((!name_window || !name_window[0]) && global_control_panel) //window name is null and we have a control panel
+    QString nameWinQt(name_window);
+
+    if (nameWinQt.isEmpty() && global_control_panel) //window name is null and we have a control panel
         layout = global_control_panel->myLayout;
 
     if (!layout)
     {
-        QPointer<CvWindow> w = icvFindWindowByName(QLatin1String(name_window));
+        QPointer<CvWindow> w = icvFindWindowByName(nameWinQt);
 
         if (!w)
             CV_Error(CV_StsNullPtr, "NULL window handler");