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.

[TOPOLOGY]
NetName=None
Nodes=0x01
Node2Present=0x01
Node2Name=SlaveNode
Node2DCFName=Slave_0_0.eds
EDSBaseName=eds