SModelRoot is empty in CommandExecutionContext

I’ve been trying to define an action similar to ApplyMarkersAction. However when attempting to fetch the model elements I noticed that the context root is empty.
The action is dispatched from within the initializeSprotty() method of a custom GLSPDiagramWidget via this.actionDispatcher.dispatch(new ApplyDiffAction());. I assume everything is bound/registered properly, as the execute() of my command is being called and the diagram is being displayed in the widget.
How can I get the root filled so that I can fetch the model elements by their ids?