'Sunshine' flatpak causes mintinstall to error out, leaving the install button loading forever

Expected Behavior

• I can open mintinstall
• I search for 'Sunshine'
• I click and open the app page for 'Sunshine' with out any problems
• I can click install on 'Sunshine'
• Sunshine gets installed

Actual Behavior

• I can open mintinstall
• I search for 'Sunshine'
• I click 'Sunshine' in the results
• mintinstall freezes for a split second
• The app page opens
• The install button is not there, instead replaced by an infinite loading throbber.

Video Representation

https://github.com/user-attachments/assets/911252fd-a9b2-43a1-9159-d816dc85a8b8

Relevant Logs

MintInstall: No new reviews
MintInstall: Deleting old screenshots
Mintinstall startup took 1723.640 ms
Traceback (most recent call last):
  File "/usr/lib/linuxmint/mintinstall/mintinstall.py", line 2422, in on_app_row_activated
    self.show_package(row.pkginfo, previous_page)
  File "/usr/lib/linuxmint/mintinstall/mintinstall.py", line 2841, in show_package
    self.task_cancellable = self.installer.select_pkginfo(pkginfo,
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 392, in select_pkginfo
    task.set_version(self)
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 146, in set_version
    self.version = installer.get_version(self.pkginfo)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/mintcommon/installer/installer.py", line 593, in get_version
    return pkginfo.get_version(as_pkg)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/mintcommon/installer/pkgInfo.py", line 385, in get_version
    version = as_pkg.get_version()
              ^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/mintcommon/installer/appstream_pool.py", line 202, in get_version
    timestamp = int(release.get_attr("timestamp"))
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

System Information

Kernel: 6.8.0-59-generic arch: x86_64
Desktop: Cinnamon v: 6.4.8 Distro: Linux Mint 22.1 Xia

Mintinstall version: 8.3.7

Start with the traceback path in mintcommon/installer/appstream_pool.py, especially get_version, then follow the calls through mintcommon/installer/pkgInfo.py and installer.py. Reproduce the Sunshine app-page flow in mintinstall and verify that the page finishes loading with an install button instead of leaving an infinite throbber.