SwiftUI, ModelContext throwing layout engine exception

A class which holds a list of spells.

@MainActor
class ListOfSpells : ObservableObject {
	@Published var allSpells : [String] = []
	
	@Published var currSpell : SpellInfo
	
	@Published var spell : String = "" {
		didSet {
			Task {
				if let spl = await DBAccess.instance.GetSpellInfo(spell: self.spell) {
					currSpell = SpellInfo(info: spl)
				}
				else {
					currSpell = SpellInfo(info: dummySpellField)
				}
			}
			
		}
	}
	
	@Published var filterString = ""
	
	init() {
		currSpell = SpellInfo(info: dummySpellField)
		
		Task {
			do {
				allSpells = try await DBAccess.instance.GetAllNames(force: false)
			}
			catch let E {
				print(E.localizedDescription)
			}
		}
	}
}

SwiftUI file

@Model
final class ThemeSpell {
	var spell : String
	
	init(spell S: String) {
		spell = S
	}
}

struct ContentView: View {
    @Environment(\.modelContext) private var modelContext
	@StateObject var spells = ListOfSpells()
	@Query private var themeSpells : [ThemeSpell]

    var body: some View {
		HStack {
			List(selection: $spells.spell) {     **
			//List {                             **
				ForEach(spells.allSpells, id: \.self) { spell in
					Text(spell)
				}
			}.frame(width: 240)
				
			
			Spacer()
			
			VStack {
				//SpellUIView(spell: spells)
				
				HStack {
					Button(">>>") {
						addItem(name: spells.spell)
					}
					.frame(width: 80)
					.keyboardShortcut(.space, modifiers: [.option])
				}
				
			}
			.frame(height: 600)
			
			Spacer()
			
			List {
				ForEach(themeSpells) { item in
					Text(item.spell)  ***
				}
			}.frame(width: 240)
			
		}
    }

	private func addItem(name: String) {
		let newItem = ThemeSpell(spell: name)
		modelContext.insert(newItem)
    }
}

** commenting out selection:$spells.spell makes no difference.

*** if this is left in, the app crashes with the following exception:

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread.'

Can't post entire stack trace. Too big. But the exception is raised a background thread.

	0   CoreFoundation                      0x000000018dd0c8c0 __exceptionPreprocess + 176
	1   libobjc.A.dylib                     0x000000018d805eb4 objc_exception_throw + 60
	2   CoreAutoLayout                      0x000000019640e240 -[NSISEngine tryToOptimizeReturningMutuallyExclusiveConstraints] + 0
	3   CoreAutoLayout                      0x000000019640e63c -[NSISEngine withBehaviors:performModifications:] + 36
	4   AppKit                              0x0000000192193fac -[NSView(NSConstraintBasedLayoutInternal) _withAutomaticEngineOptimizationDisabled:] + 48
	5   AppKit                              0x00000001921936a4 __60-[NSView(NSConstraintBasedLayoutInternal) _setLayoutEngine:]_block_invoke + 328
	6   AppKit                              0x0000000192193510 -[NSView(NSConstraintBasedLayoutInternal) _setLayoutEngine:] + 492
	7   AppKit                              0x00000001914cf998 make_and_host_engine + 144
	8   AppKit                              0x0000000192192cf4 -[NSView(NSConstraintBasedLayoutInternal) _didChangeHostsAutolayoutEngineTo:] + 56
	9   AppKit                              0x0000000192193ac8 -[NSView(NSConstraintBasedLayoutInternal) _tryToEncapsulateSubtreeLayout] + 100
	10  AppKit                              0x00000001914fae9c -[NSTableRowData _initializeRowView:atRow:] + 192
	11  AppKit                              0x00000001914f9b74 -[NSTableRowData _preparedRowViewForRow:storageHandler:] + 120
	12  AppKit                              0x000000019202d1bc __65-[NSTableRowData _doAutomaticRowHeightsForInsertedAndVisibleRows]_block_invoke_2 + 160
	13  Foundation                          0x000000018ed95704 __NSINDEXSET_IS_CALLING_OUT_TO_A_BLOCK__ + 24
	14  Foundation                          0x000000018f494fbc __NSIndexSetEnumerateBitfield + 348
	15  AppKit                              0x000000019202cf1c __65-[NSTableRowData _doAutomaticRowHeightsForInsertedAndVisibleRows]_block_invoke + 424
	16  AppKit                              0x000000019202d440 -[NSTableRowData _keepTopRowStableAtLeastOnce:andDoWorkUntilDone:] + 256
	17  AppKit                              0x00000001915082e8 -[NSTableRowData _updateVisibleViewsBasedOnUpdateItems] + 1300
	18  AppKit                              0x00000001914f0a6c -[NSTableRowData endUpdates] + 328
	19  AppKit                              0x00000001914f08e0 -[NSTableView _endUpdateWithTile:] + 108

My SwiftUI code is a modification of the standard code created with New Project. The standard code uses a toolbarItem '+' to create a new item. My code uses a list of strings to which can be added to the model.