wxPython4 sequels: fix exception in connection dialog, force black text in FBD.

- wxPython4 does not have RemoveSizer in API anymore.
- On GTK3 with dark theme default text colour is white, making it unreadable.

Beremiz's user manual
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Contents:

.. toctree::
   :maxdepth: 2

   install
   start
   edit
   build
   connectors
   debug