Layers - Prevent brushes from going null when provider is missing

Layers - Set broken state when brush provider is missing
UI - Tweaked broken state visuals

src/Artemis.Core/Models/BreakableModel.cs

@@ -59,7 +59,7 @@ public abstract class BreakableModel : CorePropertyChanged, IBreakableModel
     }
 
     /// <inheritdoc />
-    public void SetBrokenState(string state, Exception? exception)
+    public void SetBrokenState(string state, Exception? exception = null)
     {
         BrokenState = state ?? throw new ArgumentNullException(nameof(state));
         BrokenStateException = exception;