Sentry: check if post still exists before trying to change reactions

to reproduce:

• open a forum thread in two tabs
• delete posts in one tab
• attempt to modify reactings in the other tab

https://sentry.io/organizations/foodsharing-ev/issues/1964367636

see also the Slack discussion about how to handle the case of missing values where they would be expected most of the time:

• https://yunity.slack.com/archives/C1T1ZM9A4/p1607244171004400