I tested netplan.io 0.104-0ubuntu2~20.04.4 from focal-proposed, TestCase2 "Classic". Everything worked as expected, no crashes observed. Also the new "test_try_accept_lp1959570" autopkgtest PASSed on all architectures.

root@ff-sru:~# dpkg -l | grep netplan                                                                                  
ii  libnetplan0:amd64              0.104-0ubuntu2~20.04.4            amd64                                                                                                 
ii  netplan.io                     0.104-0ubuntu2~20.04.4            amd64

root@ff-sru:~# apt install network-manager
root@ff-sru:~# mv /usr/bin/nmcli /snap/bin/nmcli
root@ff-sru:~# netplan get                                                                                     [16/358]
network:                                                                                                               
  version: 2                                                                                                           
  ethernets:                                                                                                           
    eth0:                                                                                                              
      dhcp4: true                                                                                                      
root@ff-sru:~# netplan set network.renderer=NetworkManager # change global renderer to NM                              
root@ff-sru:~# netplan set network.bridges.br54.addresses=[10.0.0.20/24]                                               
root@ff-sru:~# netplan get                                                                                             
network:                                                                                                               
  version: 2                                                                                                           
  renderer: NetworkManager                                                                                             
  ethernets:                                                                                                           
    eth0:                                                                                                              
      dhcp4: true                                                                                                      
  bridges:                                                                                                             
    br54:                                                                                                              
      addresses:                                                                                                       
      - "10.0.0.20/24"
root@ff-sru:~# env -u PATH /usr/sbin/netplan try # Try-Apply the configuration
Do you want to keep these settings?


Press ENTER before the timeout to accept the new configuration


Changes will revert in 115 seconds
Configuration accepted.
root@ff-sru:~# /snap/bin/nmcli d show br54
GENERAL.DEVICE:                         br54
GENERAL.TYPE:                           bridge
GENERAL.HWADDR:                         AE:E7:C5:00:4C:D2
GENERAL.MTU:                            1500
GENERAL.STATE:                          100 (connected)
GENERAL.CONNECTION:                     netplan-br54
GENERAL.CON-PATH:                       /org/freedesktop/NetworkManager/ActiveConnection/2
IP4.ADDRESS[1]:                         10.0.0.20/24
IP4.GATEWAY:                            --
IP4.ROUTE[1]:                           dst = 10.0.0.0/24, nh = 0.0.0.0, mt = 425
root@ff-sru:~# ip addr show br54
2: br54: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether ae:e7:c5:00:4c:d2 brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.20/24 brd 10.0.0.255 scope global noprefixroute br54
       valid_lft forever preferred_lft forever


root@ff-sru:~# killall netplan-dbus
netplan-dbus: no process found
root@ff-sru:~# env -u PATH /usr/lib/netplan/netplan-dbus &
[1] 2844
root@ff-sru:~#  busctl call io.netplan.Netplan /io/netplan/Netplan io.netplan.Netplan Config
o "/io/netplan/Netplan/config/CFHCE2"
root@ff-sru:~# busctl call io.netplan.Netplan /io/netplan/Netplan/config/CFHCE2 io.netplan.Netplan.Config Try u 120
b true
root@ff-sru:~# busctl call io.netplan.Netplan /io/netplan/Netplan/config/CFHCE2 io.netplan.Netplan.Config Apply
b true
root@ff-sru:~# fg
env -u PATH /usr/lib/netplan/netplan-dbus
^C