ModelCollectionEvent: allow removing event handlers from within an event handler

src/Main/Base/Project/Dom/IModelCollection.cs

@@ -21,6 +21,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 		
 		public void AddHandler(ModelCollectionChangedEventHandler<T> handler)
 		{
+			if (handler != null)
 				_handlers.Add(handler);
 		}
 		
@@ -31,12 +32,10 @@ namespace ICSharpCode.SharpDevelop.Dom
 		
 		public void Fire(IReadOnlyCollection<T> removedItems, IReadOnlyCollection<T> addedItems)
 		{
-			foreach (var handler in _handlers) {
-				if (handler != null) {
+			foreach (var handler in _handlers.ToArray()) {
 				handler(removedItems, addedItems);
 			}
 		}
-		}
 		
 		public bool ContainsHandlers
 		{