Kerbal Space Program

Ever since the SEQ-3 Cargo Storage Unit was moved from Breaking Ground to the core game with the 1.11.1 update, it has been impossible to add a SEQ-3 to a craft in the VAB or SPH. The SEQ-3 can be picked up from the parts menu without problems, but, when you attempt to attach it to a craft...

1. Upon clicking to attach the SEQ-3, said SEQ-3 disappears (but the Engineer's Report panel still shows the mass and partcount that the SEQ-3 would have added to the craft), and the game throws an Exception: NullReferenceException (visible in the Alt-F12 console).
2. While the craft is in this state:
   • Picking up any part while the craft is in this state triggers an NRE storm that continues until you set the part down.
   • No more parts can be attached to the craft; attachment nodes fail to generate on the parts you try to add (making it impossible to node-attach these parts), while, if you attempt to surface-attach parts, the part turns green-tinted in the normal manner, but fails to actually attach when you click to attach it (the console reports that the part was successfully attached, but the part still doesn't attach).
   • Switching to the "Actions", "Crew", or "Cargo" tab causes an additional NRE, and:
     • In the "Actions" tab, attempting to add or remove an action from an action group fails, and generates yet another NRE.
     • In the "Crew" tab, both the "Parts with crew capacity" and "Available crew" menus are completely blank, even if you have available crew and the craft has crewable parts.
     • In the "Cargo" tab, the menu showing the craft's available cargo slots and their current cargo loads is completely blank, even if the craft has parts with cargo capacity.
   • Attempting to switch editors fails, and generates an NRE.
   • If you try to leave the editor, and click the "Don't Save" button on the are-you-sure dialog, it fails, and (again) generates an NRE.
3. Undoing the attempted SEQ-3 addition with Ctrl-Z restores normal behaviour, as long as you don't try adding a SEQ-3 again.

This occurs no matter whether you're in the VAB or the SPH.

Weirdly, this doesn't occur if you use a SEQ-3 to start a vessel; a SEQ-3 used as the first part of a vessel behaves perfectly normally (any subsequent SEQ-3s you try to add, however, will fall prey to the bug).