Related to issue3592 at least when it comes to setup for the reproducing steps.
- ensure you have a *.gui file with a Button component having actionEvent="true"
- start GUIBuilder
- switch to Buttons Properties Inspector Events -> there is no "x" to delete the Action Event
- change the Buttons Text property
- save and exit GuiBuilder
- check the *.gui file -> actionEvent is gone
Actually the bug is already visible in 3., though, this may go unrecognized for most users.