More info 
https://codeberg.org/Serroda/fluid-tile/issues/30

This could be the problem 
https://invent.kde.org/plasma/kwin/-/blame/master/src/effect/globals.h#L61

```js
workspace.windowAdded.connect(window => {
    window.maximizedAboutToChange.connect(mode => console.log("whatsup", mode));
});
```

works for me

```
js: whatsup 3
js: whatsup 0
```

can you attach a simple script to the bug report that reproduces the issue?

The error occurs occasionally. I can't tell you exactly when it happens; it just shows the error in the log. 

```js
window.maximizedAboutToChange.connect((mode) => {
      onMaximizeChange(mode, window);
    });
  }

  function onMaximizeChange(mode, window) {
    if (mode !== 0) {
      return;
    }
    //DO STUFF
  }
```

Maybe the `window` object is not valid or something? If kwin crashes, can you share a backtrace please?

More logs

```
Dec 16 02:04:35 kwin_wayland[1649087]: QDBusConnection: couldn't handle call to Teardown, no slot matched
Dec 16 02:04:35 kwin_wayland[1649087]: Could not find slot Krunner1Adaptor::Teardown
Dec 16 02:05:45 kwin_wayland[1649087]: "Could not convert argument 0 from undefined to QList<qulonglong>"
Dec 16 02:05:45 kwin_wayland[1649087]:          "onWindowMaximizedStateChanged@:99"
Dec 16 02:05:45 kwin_wayland[1649087]: <Unknown File>:99: TypeError: Passing incompatible arguments to C++ functions from JavaScript is not allowed.
```
`MaximizeMode` was `undefined` 
- "Could not convert argument 0 from undefined to QList<qulonglong>"

`Window` object is valid and not `undefined`

This only happens in specific cases, when the window changes state to maximized but does not return a `mode`

Ok nevermind is a problem with my code, `maximizedAboutToChange` is working 
Sorry!