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