Widget preview: has to be refreshed when you switch a calendar

[troughc]

Sometimes the widget preview is empty when you first go to the Listing widget tab.
You can then refresh the browser and the events will show up.
Expected behaviour: events should appear when I open the Listing widget tab (if there are events in the selected calendar). I know of one case for sure that causes this bug to appear
Steps to reproduce:

1. Display the Listing widget tab (in Settings) for a calendar, for example, Tout Culture.
2. Select any other calendar from the dropdown, for example, select Signe Laval.
3. Display the Listing widget tab, in settings.
4. The preview displays no results found (see screenshot).
5. Refresh the browser. The events appear in the preview.