Memory leak from toolbar button

Noticed this in someone's logs:

```
[LOG 16:06:37.377] [KSPCF:MemoryLeaks] A destroyed ProbeControlRoomRecontrolled:ProbeControlRoom instance is owning a onGUIApplicationLauncherReady callback. No action has been taken, but unless this mod is relying on this pattern, this is likely a memory leak.
```