SUMMARY

The Firefox integration extension is not checking a port status before using it,
which makes journalctl logs very cluttered.


STEPS TO REPRODUCE
1. Run firefox with the plasma integration app
2. Check journalctl

OBSERVED RESULT

Logs are cluttered with,
```
│Aug 02 11:44:57 (hostname) xsession[3458]: JavaScript error: moz-extension://41f4e5f9-376d-4fcf-aa73-f662502f23bc/extension-utils.js, line 0: Error: Attempt to postMessage on disconnected port
```

ISSUE

I see that moz-extension://41f4e5f9-376d-4fcf-aa73-f662502f23bc/extension-utils.js
calls `port.postMessage` in 2 places, both unchecked,

```

function sendPortMessage(subsystem, event, payload)
{
    // why do we put stuff on root level here but otherwise have a "payload"? :(
    var message = payload || {}
    message.subsystem = subsystem;
    message.event = event;

    port.postMessage(message);
    ///  ^^^^^^^^^^^
    ///  line 0: Error: Attempt to postMessage on disconnected port
}

function sendPortMessageWithReply(subsystem, event, payload)
{
    return new Promise((resolve, reject) => {
        let message = payload || {};
        message.subsystem = subsystem;
        message.event = event;
        ++currentMessageSerial;
        if (currentMessageSerial >= Math.pow(2, 31) - 1) { // INT_MAX
            currentMessageSerial = 0;
        }
        message.serial = currentMessageSerial;

        port.postMessage(message);  ///   <-------
        ///  ^^^^^^^^^^^
        ///  line 0: Error: Attempt to postMessage on disconnected port

        pendingMessageReplyResolvers[message.serial] = resolve;
    });
}
```

Also, maybe the port disconnection is itself also a bug, but even if it were not
the defensive checks should still be in place.

EXPECTED RESULT

  * No verbose errors are being reported.
  * The postMessage calls check for the port status prior to using it.


SOFTWARE/OS VERSIONS

Operating System: NixOS 20.09pre236419.a45f68ccac4
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.71.0
Qt Version: 5.12.7
Kernel Version: 5.4.54