Reproduction steps:
1. Open the attached project "Bug_CtrlZ.zip";
2. If the custom editor panel is not there, go to Window -> LevelUtils to open it;
3. Select the "BUG" button in the panel;
4. Press CTRL+Z;
5. Try to close the editor window;

Expected results: The editor will be closed successfully;
Actual results: The editor crashes on closing.

Reproduced on: 5.5.3p3, 5.6.1f1, 2017.1.0b8, 2017.2.0a1;
Not reproduced on: 5.4.4p3.
Regression firstly introduced: 5.5.