Can't choose future civic on civic tree

When 'always open tech/civic tree' option is enabled in CQUI, future civic (probably also tech) can't be choosen for another research.
Possible fix would be auto disabling this option when end of tech/civic tree is reached or always un-marking last civic/tech after completion.

Reproduce the issue in CQUI by enabling “always open tech/civic tree,” completing the tree, and trying to choose a future civic or technology. Inspect the research-selection UI involved; done means the option is disabled at the end of the tree or the completed final item is no longer selected, so another research can be chosen.

Written by the indexing model from the issue text.