Show Function Bar as default on NULL

[BenBE]

This is an idea I had while trying to fix up the memory leak in PR #745. This way only the relevant function bar has to be set when needed (of the two options needed for the controls) and can just be reset whenever the default should be used.

Note: This initial version of the PR has a UAF on exit when the incremental search was triggered at least once. Please take a look where I'm missing something.