pcbnew: adding non-copper Zone cutout results in crash

Description

Steps to reproduce

1. Open pcbnew
2. Draw new non-copper zone for example on F.Mask layer
3. Draw a zone cutout in the zone (press shift-c when the zone is selected or use context menu)
4. KiCad crashes, but only in like 9 out of 10 tries:

terminate called after throwing an instance of 'std::out_of_range'
  what():  map::at
fish: “kicad” terminated by signal SIGABRT (Abort)

KiCad Version

Application: Pcbnew

Version: (5.99.0-3185-g4354fa7bd), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.72.0 OpenSSL/1.1.1g zlib/1.2.11 zstd/1.4.5 libidn2/2.3.0 libpsl/0.21.1 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.41.0

Platform: Linux 5.8.5-arch1-1 x86_64, 64 bit, Little endian, wxGTK

Build Info:
	Date: Sep  6 2020 05:01:07
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.72.0
	OCC: 7.4.0
	Curl: 7.72.0
	ngspice: 32
	Compiler: GCC 10.2.0 with C++ ABI 1014

Build settings:
	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_OCC=ON
	KICAD_SPICE=ON