"Check zone fills before plotting" doesn't work on first time plotting.

Description

When generating gerbers with zones unfilled "Check zone fills before plotting" doesn't work until the second time plotting.

Steps to reproduce

1.Draw a PCB with a zone and some traces. 2.Press ctrl-b to unfill the zone 3.Go to plot gerbers and click Plot (With "Check zone fills already selected") It will plot without filling the zones. 4.Clicking Plot again will query "Zone fills are out-of-date. Refill?"

KiCad Version

Application: Pcbnew
Version: 5.1.8-db9833491~87~ubuntu20.04.1, release build
Libraries:
    wxWidgets 3.0.5
    libcurl/7.68.0 OpenSSL/1.1.1f zlib/1.2.11 brotli/1.0.9 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh/0.9.3/openssl/zlib nghttp2/1.41.0 librtmp/2.3
Platform: Linux 5.8.0-29-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
    Boost: 1.71.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.68.0
    Compiler: GCC 9.3.0 with C++ ABI 1013

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=ON
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=ON
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON