Error inside fapi5 in onPostMessage?

Question

Classmates

With enough players, we get an exception inside fapi5.
TypeError: c.data.split is not a function - in the code c.data.split("$")
Example: https://app.getsentry.com/share/issue/36393037382e...
Not valid data is received in onPostMessage from outside and the data field is not a string.

Answer 1

In response to what request is such data received? And what exactly is the data?