footprint generator, qfn, checking Thermal Vias for exposed pad results in python error

1. Use QFN generator
2. Go to EPAD, check Thermal Vias
3. Instantly get the python error

Application: KiCad Footprint Editor x64 on x64

Version: (6.99.0-5337-gc57e6db79a), release build

Libraries:
	wxWidgets 3.2.1
	FreeType 2.12.1
	HarfBuzz 5.0.1
	FontConfig 2.14.1
	libcurl/7.86.0-DEV Schannel zlib/1.2.13

Platform: Windows 10 (build 19045), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Jan  6 2023 06:18:16
	wxWidgets: 3.2.1 (wchar_t,wx containers)
	Boost: 1.80.0
	OCC: 7.6.2
	Curl: 7.86.0-DEV
	ngspice: 38
	Compiler: Visual C++ 1934 without C++ ABI

Build settings:
	KICAD_SPICE=ON