Allow to set null to creator.selectedElement

[andrewtelnov]

It is currently impossible to set creator.selectedElement to null. It would select the survey and as result show survey properties in the property.
By allowing to set the null value to creator.selectedElement, it will become possible do not show survey/page properties in the property grid. Here is the example that disable selecting survey and page:

      creator.onSelectedElementChanging.add(function(c, options) {
        var el = options.newSelectedElement;
        if (
          el != null &&
          (el.getType() == "page" || el.getType() == "survey")
        ) {
          options.newSelectedElement =
            creator.survey.getAllQuestions().length > 0
              ? creator.survey.getAllQuestions()[0]
              : null;
        }
      });

      //creator.JSON = YourSurveyJSON;
      creator.selectedElement = null;