Crash on exit if layouts have been renamed

[klayoutmatthias]

To reproduce:

run this script:

mw = RBA::Application::instance.main_window

mw.create_layout(1)
cv = RBA::CellView::active
cv.name = "L1"
mw.create_layout(2)
cv = RBA::CellView::active
cv.name = "L1"

mw.create_layout(1)
cv = RBA::CellView::active
cv.name = "L2"
mw.create_layout(2)
cv = RBA::CellView::active
cv.name = "L2"
mw.create_layout(2)
cv = RBA::CellView::active
cv.name = "L2"

mw.create_layout(1)
cv = RBA::CellView::active
cv.name = "L1"
mw.create_layout(2)
cv = RBA::CellView::active
cv.name = "L3"
mw.create_layout(2)
cv = RBA::CellView::active
cv.name = "L3"

then close one tab and close the application. The application crashes.