For those still watching this thread: the memory accesses were correct, valgrind just couldn't handle the jemalloc allocator. Using the system allocator instead, even destroying the window before using it does not lead to invalid accesses.
Isn't it still undefined behaviour to do so, though?
Valgrind doesn't detect anything, so I guess GTK's reference counting does the right thing and doesn't prematurely free data still referenced somewhere else. It just doesn't show a window.