Mint 21.1 Vera: mintinstall crash if flatpak is not installed

In Linux Mint 21.1 Vera the Mintinstall Software Manager no longer works if flatpak is not installed.

That's too bad, not everyone wants to have flatpak support packages installed on their system. Please add some check to disable flatpak support in mintinstall if flatpak is not installed on the system.

$ mintinstall
MintInstall: Detected system architecture: 'x86_64'
Traceback (most recent call last):
  File "/usr/lib/linuxmint/mintinstall/mintinstall.py", line 958, in do_activate
    if self.installer.init_sync():
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 246, in init_sync
    if self._fp_remotes_have_changed():
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 326, in _fp_remotes_have_changed
    fp_remotes = self.list_flatpak_remotes()
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 492, in list_flatpak_remotes
    return _flatpak.list_remotes()
  File "/usr/lib/python3/dist-packages/mintcommon/installer/_flatpak.py", line 897, in list_remotes
    fp_sys = get_fp_sys()
  File "/usr/lib/python3/dist-packages/mintcommon/installer/_flatpak.py", line 72, in get_fp_sys
    _fp_sys = Flatpak.Installation.new_system(None)
gi.repository.GLib.GError: g-io-error-quark: While opening repository /var/lib/flatpak/repo: opening repo: opendir(/var/lib/flatpak/repo): No such file or directory (1)
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 153, in apport_excepthook
    with os.fdopen(os.open(pr_filename,
FileNotFoundError: [Errno 2] No such file or directory: '/var/crash/_usr_lib_linuxmint_mintinstall_mintinstall.py.1000.crash'

Original exception was:
Traceback (most recent call last):
  File "/usr/lib/linuxmint/mintinstall/mintinstall.py", line 958, in do_activate
    if self.installer.init_sync():
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 246, in init_sync
    if self._fp_remotes_have_changed():
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 326, in _fp_remotes_have_changed
    fp_remotes = self.list_flatpak_remotes()
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 492, in list_flatpak_remotes
    return _flatpak.list_remotes()
  File "/usr/lib/python3/dist-packages/mintcommon/installer/_flatpak.py", line 897, in list_remotes
    fp_sys = get_fp_sys()
  File "/usr/lib/python3/dist-packages/mintcommon/installer/_flatpak.py", line 72, in get_fp_sys
    _fp_sys = Flatpak.Installation.new_system(None)
gi.repository.GLib.GError: g-io-error-quark: While opening repository /var/lib/flatpak/repo: opening repo: opendir(/var/lib/flatpak/repo): No such file or directory (1)

Reproduce the crash by launching mintinstall on a system without Flatpak support. Start with mintinstall.py at do_activate, then follow installer.py and installer/_flatpak.py through init_sync(), _fp_remotes_have_changed(), and get_fp_sys(). Done means Mintinstall opens and disables or skips Flatpak support when /var/lib/flatpak/repo is absent.