so I've recently been learning altium and I'm one of those learners who tends to click things to see what they do and then figure out how to undo it afterwards. Unfortunately I've now run into a problem with that method because at some point I clicked the "rigid/flex design" option in the features tab of the layer stack manager and have managed to save the whole project in a state where I can't delete that substack.

I open the layer stack manager, deselect the "rigid/flex" option and it returns to a normal stack up, complete with a bottom solder mask layer (which I'm now trying to add) and save the stack. But when I reopen the layer stack manager, the substack is there, and the rigid/flex option is still active and I can't add a bottom solder mask to the pcb. 

I can't find anything on the internet that say's anything other than "just delete the substack" but the problem is I can't delete it at all, the bin icon is greyed out. 

Does anyone know of a way to undo that without just recreating the project? I'm worried about misaligning screw holes and connector tabs and the like and I'd rather know what I did so I can undo it in future rather than just nuke the thing and start again.