Crash with multiple windows open, unknown cause

Steps to reproduce:

1. I had 2-4 windows open (of the same file, bg.svg from inkscape#781)
2. I believe while closing one of the windows, the crash happened

What happened?

Crash, coredump attached. There is one immediate fix apparent in the backtrace, but there may be a bigger problem here.

Potential patch MERGED inkscape@66d3f78b . Doesn't really identify the cause though, and may keep Inkscape in buggy state.

Backtrace: inkscape-backtrace-unknown.txt

Truncated backtrace:

#0  Inkscape::UI::Widget::Canvas::forced_redraws_stop (this=0x0) at ../src/ui/widget/canvas.h:100
100	    void forced_redraws_stop() { _forced_redraw_limit = -1; }
[Current thread is 1 (Thread 0x7fbca508fb40 (LWP 227380))]
(gdb) bt
#0  Inkscape::UI::Widget::Canvas::forced_redraws_stop() (this=0x0) at ../src/ui/widget/canvas.h:100
#1  Inkscape::Rubberband::stop() (this=0x556f7dab8fd0) at ../src/rubberband.cpp:74
#2  0x00007fbcacf9ed10 in Inkscape::UI::Tools::SelectTool::root_handler(_GdkEvent*) (this=0x556f835a6ae0, event=0x556f8095cda0) at ../src/ui/tools/select-tool.cpp:785
#3  0x00007fbcacfbc8fb in Inkscape::UI::Tools::sp_event_context_virtual_root_handler(Inkscape::UI::Tools::ToolBase*, _GdkEvent*)
    (event_context=event_context@entry=0x556f835a6ae0, event=event@entry=0x556f8095cda0) at ../src/ui/tools/tool-base.cpp:1161

(gdb) frame 1
#1  Inkscape::Rubberband::stop (this=0x556f7dab8fd0) at ../src/rubberband.cpp:74
warning: Source file is more recent than executable.
74	        _desktop->getCanvas()->forced_redraws_stop();
(gdb) p _desktop
$1 = (SPDesktop *) 0x556f7d825540
(gdb) p _desktop->canvas
$2 = (Inkscape::UI::Widget::Canvas *) 0x0

(gdb) frame 2
#2  0x00007fbcacf9ed10 in Inkscape::UI::Tools::SelectTool::root_handler (this=0x556f835a6ae0, event=0x556f8095cda0) at ../src/ui/tools/select-tool.cpp:785
785	                Inkscape::Rubberband::get(desktop)->stop(); // might have been started in another tool!
(gdb) p desktop
$1 = (SPDesktop *) 0x556f7aeeca80


(gdb) frame 17
#17 0x00007fbcad267ea2 in Inkscape::UI::Widget::Canvas::emit_event (this=this@entry=0x556f79e26000, event=event@entry=0x556f80954220)
    at ../src/ui/widget/canvas.cpp:1695
1695	            finished = item->handle_event(event_copy);
(gdb) p *this
(gdb) p _desktop
$5 = (SPDesktop *) 0x556f7aeeca80
(gdb)  

What should have happened?

No crash

Version Info:

• Inkscape 1.1-dev (9e41ff21b6, 2020-12-18) Debug build Linux Mint 20

Configuration Summary
------------------------------------------------------------------------
CMAKE_SYSTEM_NAME:       Linux
CMAKE_SYSTEM_VERSION:    5.4.0-42-generic
CMAKE_SYSTEM_PROCESSOR:  x86_64
CMAKE_C_COMPILER:        /usr/bin/cc
CMAKE_CXX_COMPILER:      /usr/bin/c++
CMAKE_BUILD_TYPE:        Debug

ENABLE_LCMS:             ON
ENABLE_POPPLER:          ON
ENABLE_POPPLER_CAIRO:    ON
WITH_GNU_READLINE:       ON
WITH_DBUS:               OFF
WITH_GSPELL:             ON
WITH_IMAGE_MAGICK:       ON
WITH_GRAPHICS_MAGICK:    OFF
WITH_LIBCDR:             ON
WITH_LIBVISIO:           ON
WITH_LIBWPG:             ON
WITH_NLS:                ON
WITH_OPENMP:             ON
WITH_JEMALLOC:           OFF
WITH_INTERNAL_2GEOM:     ON
WITH_PROFILING:          OFF
BUILD_TESTING:           ON