Crash hiding scrollbar after opening/closing a few times
#### Steps to reproduce:
- open Inkscape
- Ctrl+N (new window)
- Ctrl+B (hide scrollbar)
- Ctrl+W (close document and thus window)
- Repeat last 3 steps 3-4 times. Ctrl+B will show (not hide) the scrollbar every second time
#### What happened?
- Crash on hiding scrollbar
- Backtrace in Inkscape 1.1-dev (8d201ce056, 2021-01-09) [inkscape-backtrace-hide-scrollbar.txt](/uploads/88c4087fae9481aaa999179fbe13c281/inkscape-backtrace-hide-scrollbar.txt)
- Truncated backtrace:
```
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007f08a2729ee2 in __dynamic_cast ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
[Current thread is 1 (Thread 0x7f089bcdcb40 (LWP 219009))]
(gdb) set pagination off
(gdb) bt
#0 0x00007f08a2729ee2 in __dynamic_cast () at /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#1 0x00007f08a3a6e2f8 in set_menuitems(unsigned int, bool) (emitting_verb=193, value=<optimised out>) at ../src/ui/desktop/menubar.cpp:124
... (sigc signal stuff)
#7 SPDesktop::toggleScrollbars() (this=this@entry=0x555907936540) at ../src/desktop.cpp:1346
```
- 
#### What should have happened?
- No crash
Sample attachments: [inkscape-backtrace-hide-scrollbar.txt](/uploads/9f1e2844c35d6ecead291f4e4df48513/inkscape-backtrace-hide-scrollbar.txt) and [inkscape-scrollbar](/uploads/66b80733a65858aa8cf1175db544214d/inkscape-scrollbar.mp4)
#### Version Info:
- Inkscape 1.0.1 (179fe94779, 2021-01-06) Linux Mint 20
- Inkscape 1.1-dev (8d201ce056, 2021-01-09) Linux Mint 20
Bisection points to commit inkscape@a7e562fc5b5f2c6a41548eaee85d9803cada7621
```
Author: Jabiertxof <jabier.arraiza@marker.es>
Date: Sun Jul 28 18:59:09 2019 +0200
Add Patrick improvements to verbs toggle menu items
src/desktop.cpp | 21 ++++++++++----------
src/desktop.h | 2 +-
src/object/sp-namedview.cpp | 5 +++--
src/object/sp-namedview.h | 2 +-
src/ui/desktop/menubar.cpp | 45 ++++++++++++++++++++++--------------------
src/ui/desktop/menubar.h | 1 +
src/verbs.cpp | 2 +-
src/widgets/desktop-widget.cpp | 2 +-
8 files changed, 43 insertions(+), 37 deletions(-)
```
Relevant MRs: https://gitlab.com/inkscape/inkscape/-/merge_requests/803 and subsequently https://gitlab.com/inkscape/inkscape/-/merge_requests/800
Note: earlier commits only required 2 Ctrl+N to replicate the issue.
issue