Material library: fix save

hide/view/Prefab.hx

@@ -621,10 +621,12 @@ class Prefab extends hide.view.FileView {
 		sys.io.File.saveContent(getPath(), content);
 		super.save();
 
-		for (entry in renameMatsHistory)
-			saveMatLibsRenames(entry.previousName, entry.newName, entry.prefab);
+		if (renameMatsHistory != null) {
+			for (entry in renameMatsHistory)
+				saveMatLibsRenames(entry.previousName, entry.newName, entry.prefab);
 
-		renameMatsHistory = [];
+			renameMatsHistory = [];
+		}
 	}
 
 	function saveMatLibsRenames(oldName : String, newName : String, prefab : hrt.prefab.Prefab) {