Is it possible to suppress error message "Uncaught (In promise) error: the scenario is currently locked and cannot be updated" popping out at the bottom of the screen?

Mostly that is true. But in the case of <vdl-action-execute> it returns when the Action has queued the job for execution then returns. We understand that this can cause issues but, in the case of model execution, it could be several hours before the job is done. There are some others and maybe we need to add to the documentation where Actions return while their subject is still in process.