Steps to reproduce:

1. Create host: test
2. Create 2 Zabbix trapper items with keys trap1 and trap2
3. Create triggers for each item with expressions: last(/test/trap1)=0; last(/test/trap2)=0
4. Start the server, send data to trapper item to switch triggers to a Problem state
5. Open Monitoring -> Problems
6. Select one of the problems
7. Click on the second problem to expand the menu, click Mark selected as symptoms
8. Wait for event to be updated
9. Send necessary data to switch Symptom trigger to RESOLVED status
10. Open dashboard
11. Expand the problem symptoms in Problems widget

Result:

Console error is displayed:

Uncaught TypeError: Cannot read properties of undefined (reading 'classList')
    at #onShowSymptoms (class.widget.js?1723181596:124:15)
    at HTMLButtonElement.<anonymous> (class.widget.js?1723181596:90:62)

See screenshot:

Expected:
Should be possible to expand the symptom list in the Problems widget, while symptom trigger is in resolving status