In version R3 2019 (2019.3.917), the following issue is observed with various date pickers:
Reproduction code and steps:
<telerik:RadDateTimePicker RenderMode="Lightweight" ID="RadDateTimePicker1" runat="server" >
</telerik:RadDateTimePicker>
1) open date picker,
2) click on the month/year to open the MonthYearFastNavigation
3) Click Ok or Cancel
Expected: date selected
Actual: JavaScript error: Uncaught TypeError: Cannot read property '_popupButton' of null