Brief summary of issue / Description of requested feature:

using setButtonStyleSheet almost always crashes Mudlet

Steps to reproduce the issue / Reasons for adding feature:

1. Create new empty profile
2. create a new button toolbar
3. create a new button named "test"
4. activate the button and the button-toolbar
5. use

lua setButtonStyleSheet("test","background-color: red;")

in the command line
6. crash

Error output / Expected result of feature

it crashes.

After testing for a while I noticed that it doesn't crash every time and if It's not crashing once it won't for a while (even after restarting Mudlet)

This makes this issue very unpredictable.

Extra information, such as Mudlet version, operating system and ideas for how to solve / implement:

Mudlet 4.9.1
Ubuntu 20.04

qt creator debugger output seems to point at ActionUnit::findActionsByName and there at action->getName()
debugger output: