Link: http://services.netbeans.org/synergy/client/app/#/case/5142/suite/2026

Product Version = NetBeans IDE Dev (Build 201508310002)
Operating System = Linux version 3.16.0-40-generic running on amd64 (Ubuntu)
Java; VM; Vendor = 1.8.0_45
Runtime = Java HotSpot(TM) 64-Bit Server VM 25.45-b02

Reproducibility: Happens every time

STEPS:
1. Open extract.ClassB.java class
2. From main menu select Refactor > Extract Interface...
3. In Extract Interface dialog type interface name MyInterface, check method m3() and click Refcator
4. Inspect changes then, right click extract.ClassB.java in Projects view and from pop-up menu select Refactor > Extract Interface...
5. In Extract Interface dialog type interface name MyInterface1, check interface MyInterface then click Refcator
6. Press two times undo again and confirm every time it's needed

ACTUAL:
Everything works as expected but Undo/Redo.
1. After Step 3, If I Undo, i see an error popup, but undo functions properly, but, after Redo this does not do anything
2. After Step 5, If I Undo, I see an error popup, but undo functions properly, but If I Undo again, I get another error saying "Really Undo null"

EXPECTED:
- MyInterface is created, ClassB implements its method m1() which is anotated with @override
- MyInterface1 is created and it extends from MyInterface
- Code is every time compilable
- Undo works properly as expecte