Simulator: signal is not removed from plot when removed from schematic

Description

After plotting a signal, if you then remove the signal from the schematic, it's removed from the signal list but the graphical plot is still there and can't be hidden, and simulator gives error in log window: Error: vector V(/signalName) not found!.

Steps to reproduce

1. Plot a signal (enable the Plot checkbox in simulator window)
2. Remove signal from schematic
3. Run simulation again
4. Observe plot is still visible, but can't be hidden/removed.

KiCad Version

Application: KiCad Schematic Editor arm64 on arm64

Version: (7.99.0-172-g343e3aee55), release build

Libraries:
	wxWidgets 3.2.1
	FreeType 2.12.1
	HarfBuzz 5.3.1
	FontConfig 2.14.0
	libcurl/7.79.1 (SecureTransport) LibreSSL/3.3.6 zlib/1.2.11 nghttp2/1.45.1

Platform: macOS Monterey Version 12.5.1 (Build 21G83), 64 bit, Little endian, wxMac

Build Info:
	Date: Feb 20 2023 01:20:28
	wxWidgets: 3.2.1 (wchar_t,wx containers)
	Boost: 1.80.0
	OCC: 7.6.3
	Curl: 7.77.0
	ngspice: 38
	Compiler: Clang 13.0.0 with C++ ABI 1002

Build settings:
	KICAD_SPICE=ON