Fix NullReferenceException in IterateViaForeachAction.

ICSharpCode.NRefactory.CSharp/Refactoring/CodeActions/IterateViaForeachAction.cs

@@ -149,11 +149,10 @@ namespace ICSharpCode.NRefactory.CSharp.Refactoring
 		{
 			IType collectionType = null;
 			foreach (var baseType in type.GetAllBaseTypes()) {
-				var baseTypeDefinition = baseType.GetDefinition();
+				if (baseType.IsKnownType(KnownTypeCode.IEnumerableOfT)) {
-				if (baseTypeDefinition.IsKnownType(KnownTypeCode.IEnumerableOfT)) {
 					collectionType = baseType;
 					break;
-				} else if (baseTypeDefinition.IsKnownType(KnownTypeCode.IEnumerable)) {
+				} else if (baseType.IsKnownType(KnownTypeCode.IEnumerable)) {
 					collectionType = baseType;
 					// Don't break, continue in case type implements IEnumerable<T>
 				}