Steps to repro:

1. Create new project;
2. Select any GameObject in the Hierarchy (for instance, Directional Light);
3. Lock Inspector tab by pressing button in the upper right corner of the tab;
4. Undock Inspector tab from main Editor window.

Actual result:
- Inspector tab is unresponsive;
- Error:

'NullReferenceException: Object reference not set to an instance of an object
UnityEditor.EditorWindow.set_position (Rect value)'

is shown in the Console window. See attached video.
Regression introduced in Unity 5.6.0b6.

Expected result:
Locked tab is undocked correctly without errors.

Notes:
- Reproducible in 5.6.0b6; Not reproducible in 5.6.0b5, 5.5.0p1;
- Reproducible on Windows 10 and OSX 10.12.2.
- Reproducible only when undocking locked tab / not reproducible when re-docking locked tab;
- Not reproducible when undocking / re-docking unlocked tab.