Crash When Carla Rack or Patchbay is Replaced

### Bug Summary
LMMS crashes or becomes unstable after Carla Rack or Patchbay instrument is replaced while its GUI is still active

#### Steps to reproduce
1. Add a Carla Rack or Patchbay instrument to the song editor.
2. Open that Carla instrument and click on _'Show GUI'_
3. Drag another instrument from the _'Instrument Plugins'_ panel and drop it on top of the existing Carla instrument
4. Sit back, grab some popcorn and watch the fireworks...
5. If LMMS doesn't immediately crash, grab another Carla instrument and add it to the song editor

*if the steps above don't result in a crash, try moving the Carla window to a different workspace.*

#### Expected behavior
The Carla Rack/Patchbay instrument gets replaced by the new instrument that was dropped on top of it.

#### Actual behavior
LMMS crashes swiftly or becomes very unstable and crashes if the user tries adding or changing more unrelated instruments.

### Note:
**Some instruments do not trigger this crash**(???)
From my testing, LB302 sometimes works fine initially, but LMMS crashes if another Carla instrument is added
Kicker sometimes works fine if the Carla window is in the same workspace.

#### Affected LMMS versions
1.2.1
1.2.1.701 (#5499)
1.3.0 alpha
1.3.0 alpha 1.44+g97114fb (https://github.com/LMMS/lmms/pull/5846#issuecomment-760939141)

#### Logs

Click to expand

~/Downloads$ ./lmms-1.3.0-alpha.1.33+g4f74151-linux-x86_64.AppImage 

Carla appears to be installed on this system at /usr/lib[64]/carla so we'll use it.

Jack appears to be installed on this system, so we'll use it.

Lv2 plugin SUMMARY: 95 of 188  loaded in 1556 msecs.

For details about not loaded plugins, please set

  environment variable "LMMS_LV2_DEBUG" to nonempty.

Lv2 Plugins blacklisted: 2 of 188 

  If you want to ignore the blacklist (dangerous!), please set

  environment variable "LMMS_IGNORE_BLACKLIST" to nonempty.

Trying to start carla-plugin using "/usr/share/carla/resources/carla-plugin-patchbay"

LRDF Support not available (LADSPA-RDF will be disabled)

Carla 2.1.0 started, status:

  Python version: 3.8.5

  Qt version:     5.12.8

  PyQt version:   5.14.1

  Binary dir:     /usr/lib/carla

  Resources dir:  /usr/share/carla/resources

ALL OK!

qt.svg: :552:178: Could not resolve property: #linearGradient5167

qt.svg: :30274: Could not resolve property: #linearGradient5167

qt.svg: :30274: Could not resolve property: #pattern5614

qt.svg: :30274: Could not resolve property: #pattern5626

qt.svg: :5672: Could not resolve property: #radialGradient3709

qt.svg: :3226: Could not resolve property: #linearGradient3893-1

qt.svg: :3190: Could not resolve property: #linearGradient3893-1

qt.svg: :3208: Could not resolve property: #linearGradient3893-1

Frontend pixel ratio is 1.0

Got SIGTERM -> Closing now

CarlaPipeCommon::_writeMsgBuffer(..., 15) - failed with -1 (true), message was:

__carla-quit__

Trying to start carla-plugin using "/usr/share/carla/resources/carla-plugin-patchbay"

LRDF Support not available (LADSPA-RDF will be disabled)

Carla 2.1.0 started, status:

  Python version: 3.8.5

  Qt version:     5.12.8

  PyQt version:   5.14.1

  Binary dir:     /usr/lib/carla

  Resources dir:  /usr/share/carla/resources

ALL OK!

qt.svg: :552:178: Could not resolve property: #linearGradient5167

qt.svg: :30274: Could not resolve property: #linearGradient5167

qt.svg: :30274: Could not resolve property: #pattern5614

qt.svg: :30274: Could not resolve property: #pattern5626

qt.svg: :5672: Could not resolve property: #radialGradient3709

qt.svg: :3226: Could not resolve property: #linearGradient3893-1

qt.svg: :3190: Could not resolve property: #linearGradient3893-1

qt.svg: :3208: Could not resolve property: #linearGradient3893-1

Frontend pixel ratio is 1.0

Got SIGTERM -> Closing now

CarlaPipeCommon::_writeMsgBuffer(..., 15) - failed with -1 (true), message was:

__carla-quit__

/tmp/.mount_lmms-1Jt8EDB/AppRun: line 24: 34297 Segmentation fault      (core dumped) QT_X11_NO_NATIVE_MENUBAR=1 "$DIR"/usr/bin/lmms.real "$@"

Start by reproducing the crash with a Carla Rack or Patchbay instrument, its Show GUI window active, and an instrument replacement from the Instrument Plugins panel; review the attached Carla and segmentation-fault log. Done means replacing the instrument no longer crashes or destabilizes LMMS, including when the Carla window is moved to another workspace.

Written by the indexing model from the issue text.