Motivation for this change

I have been experiencing errors while using FileZilla due to the bug in wxwidgets library.
Annoying pop up with error message is appearing every time FileZilla is started.

wxTopLevelWindowBase::Destroy calls Hide on itself if there is at least one other top level window.
Unfortunately calling Hide on a window that has not yet been created causes an assertion on wxGTK.
It is very easy to reproduce with this snippet you could for example add to the minimal sample in

MyFrame::OnAbout:
(new wxDialog)->Destroy();

Both WX_3_0_BRANCH and master are affected by this issue.

The error messages is here: https://gist.github.com/anonymous/38f27cbdc43c96a9c7fe063c3a10a08c

The bug is tracked here: https://trac.wxwidgets.org/ticket/17942

Patch source available here: https://trac.wxwidgets.org/attachment/ticket/17942/fix_assertion_using_hide_in_destroy.diff

I have attached a patch for this issue. With it, Hide is only called if the window is actually shown.

The attached patch has been created against WX_3_0_BRANCH but also applies cleanly against master.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • [+] NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• [+] Tested execution of all binary files (usually in ./result/bin/)
• [+] Fits CONTRIBUTING.md.