eeschema: referencing sheet variable on instantiation segfaults on load

Description

It was suggested in #10975 (closed) to use sheet variables as a way to keep instance friendly names consistent.

Unfortunately, this causes segfaults on load in 6.0.2.

Steps to reproduce

1. Start a new schematic
2. Add a hierarchical sheet
3. Add a visible property to the instantiation with text ${COMMENT1}
4. Save the design
5. Close and re-open the schematic

KiCad Version

Application: KiCad

Version: 6.0.2-378541a8eb~116~ubuntu20.04.1, release build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.68.0 OpenSSL/1.1.1f zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.2.0) libssh/0.9.3/openssl/zlib nghttp2/1.40.0 librtmp/2.3

Platform: Linux 5.13.0-28-generic x86_64, 64 bit, Little endian, wxGTK, xubuntu, x11

Build Info:
	Date: Feb 11 2022 08:14:15
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCC: 7.5.2
	Curl: 7.81.0
	ngspice: 31
	Compiler: GCC 9.3.0 with C++ ABI 1013

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON