Crash when Carla Instrument is removed

[tresf]

Removing a Carla track may cause LMMS to segfault. This was reproducible on Windows 10 using the mingw 64-bit build as well as the MSVC build.

Some notes from #5713 which originally identified this bug.

@PhysSong wrote:

Reproduced with Carla 2.2 and MSVC build of LMMS. It seems like Carla crashed in CarlaEngine::ProtectedData::deletePluginsAsNeeded() which is indirectly called from CarlaInstrument::timerEvent(). Inspecting memory and registers suggests there might be some use after free conditions.
2.1.1 Also crashes in CarlaBackend::CarlaEngineNative::uiIdle(), but less frequently.
...
I found that it's related to the (improperly implemented) InstrumentTrackWindow caching which causes a temporary CarlaInstrumentView to be instantiated after the existing one is deleted. That will be deleted after CarlaInstrument, and timer events may kick in between those two events. That results in CarlaInstrumentView::timerEvent() using a deleted handle.

Tagging for windows, however the bug may exist on other platforms.