### Problem description
Merging my previous PRs, #1660 and #1658 has resulted in a conflict
causing an application crash due to a missing `ImGui::BeginDisabled();`
in the `PopupSelect` class. Specifically, none of the code related to
offset validation made it into the final merge. This PR fixes the crash
by reintroducing the deleted lines.
### Additional things
The nightly release build seems to be unaffected by the crash, most
likely because ImGui's `DisabledStack` assertions are only enforced in
Debug mode. The offset validation was still missing before this fix.
3c91cb09e3