Add-a-risk page throws console errors

[ddohler]

Steps to reproduce:

• Click on the Vulnerability Assessment tab on the Dashboard, and then click Add a Risk
• In the Hazard box, select Air-borne disease.

Expected:
No console errors are issued

Actual:
A console error is thrown:

ERROR TypeError: "weatherEvent is null"
getWeatherEventRank    impact.service.ts:99