I took a different approach than Paul on this one. Just wanted to share and get some input about whether this is a good or bad idea.

I moved the project creation to the DataController by adding this method:

func createProject() {
        let project = Project(context: container.viewContext)
        project.closed = false
        project.creationDate = Date()
        save()
    }

Then I call that in the ProjectsView:

Button {
     withAnimation {
        dataController.createProject()
     }
} label: {
     Label("New Project", systemImage: "plus")
}

I did the same with the item creation:

func createItem(for project: Project) {
        let item = Item(context: container.viewContext)
        item.project = project
        item.creationDate = Date()
        save()
    }

I found that this approach meant that I didn't need to use an @Environment(.managedObjectContext) property in my views and perform all tasks through the DataController. This also felt more consistent with the delete functionality.