global_container_stack.definitionChanges always returns a container, global_container_stack.findContainer({"type": "definition_changes"}) does not (because an empty container does not have the type "definition_changes".
Recently we changed the empty containers such that there is only one empty container instance and it doesn't have the proper type any more. Instead we have properties on the stack that allows us to find the container with the proper type. It's faster and easier to use.
We've had a few bugs about this so I decided to update all of them to remove those for the future, except the ones in plugins/MachineSettingsAction/MachineSettingsAction.py because we have a pending pull request that fixes those. Fixing them would give merge conflicts for fieldOfView.
It doesn't really belong to CURA-4024 but I'm sticking it under that nomer anyway to get it reviewed.
Except when we fill the instancecontainer via deserialising. Because then we want to retain the original version number.
Contributes to issue CURA-3427.
