Safari 15 is (randomly) killing WkWebView instance

I have a Safari App Extension that uses an instance of WkWebView to run JavaScript code. This app extension runs flawlessly with Safari 13 & 14, but Safari 15 occasionally kills the WkWebView instance for no apparent reason. The only thing logged to the console is this message:

(JavaScriptCore) Received an invalid message "<unknown message receiver>.<unknown message>" from the web process.
SafariExtension: (WebKit) [com.apple.WebKit:Process] 0x10c2e9300 - WebProcessProxy didClose (web process crash)
SafariExtension: (WebKit) [com.apple.WebKit:Process] 0x7ff1e7048020 - [pageProxyID=5, webPageID=6, PID=22131] WebPageProxy::processDidTerminate: (pid 22131), reason 3

I can't find any correlation between the JavaScript that's running and that error. The error seemingly occurs at random.

Has anyone else experienced such a problem?

Thanks