_normalizeCurrentPicker() {
      if (this._currentPicker === "day" && this._pickersMode !== _CalendarPickersMode.default.DAY_MONTH_YEAR) {
        this._currentPicker = "month";
      }
      if (this._currentPicker === "month" && this._pickersMode === _CalendarPickersMode.default.YEAR) {
        this._currentPicker = "year";
      }
    }
Makes sure that _currentPicker is always set to a value, allowed by _pickersMode