[kicad-cli] crash when processing schematic that references title block variables

Description

If you reference a title block variable from a sheet instance, kicad is fine but kicad-cli segfaults.

gdb says it all

#3  0x00007ffff1e8ad8b in SCHEMATIC::ResolveTextVar (this=0x555555a94720, aSheetPath=0x7fffffffba20, 
    token=0x7fffffffbae0, aDepth=<optimized out>) at eeschema/schematic.cpp:271
271	    if( aSheetPath->LastScreen()->GetTitleBlock().TextVarResolver( token, m_project ) )
(gdb) print aSheetPath->LastScreen()
$2 = (SCH_SCREEN *) 0x0

Steps to reproduce

1. Extract this project: cli_crash.zip
2. Run kicad-cli sch export pdf test.kicad_sch
3. boom

KiCad Version

Application: KiCad Schematic Editor x86_64 on x86_64

Version: 8.0.1-138-ga155dd32e8, release build

Libraries:
	wxWidgets 3.2.4
	FreeType 2.11.1
	HarfBuzz 2.7.4
	FontConfig 2.13.1
	libcurl/7.81.0 OpenSSL/3.0.2 zlib/1.2.11 brotli/1.0.9 zstd/1.4.8 libidn2/2.3.2 libpsl/0.21.0 (+libidn2/2.3.2) libssh/0.9.6/openssl/zlib nghttp2/1.43.0 librtmp/2.3 OpenLDAP/2.5.17

Platform: Ubuntu 22.04.4 LTS, 64 bit, Little endian, wxGTK, X11, xubuntu, x11

Build Info:
	Date: Apr 11 2024 10:25:36
	wxWidgets: 3.2.1 (wchar_t,wx containers) GTK+ 3.24
	Boost: 1.74.0
	OCC: 7.6.3
	Curl: 7.81.0
	ngspice: 36
	Compiler: GCC 11.4.0 with C++ ABI 1016

Build settings: