I have certainly something wrong in the design file, but I cannot repair it.
I have added many windows and doors to a building, and since this time, several things do not work properly.
If you try to delete the empty group 'Piscine', other components are deleted, when toggling visibility of group it toggles visibility of the other objects as well.
Is this a bug ?

OS: Windows XP
Word size of OS: 32-bit
Word size of FreeCAD: 32-bit
Version: 0.16.6647 (Git)
Build type: Release
Branch: master
Hash: 9299a73cf1482db4120b94543b33c2f9ec758319
Python version: 2.7.8
Qt version: 4.8.6
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17