Updates dialog breaks server in some cases on Linux

[UnknownShadow200]

Mostly observed with recent Gnome DE + Ubuntu

A short time after, the server will die with a message like this

Issue seems to be due to the way Updates MessageBox is spawned from a background thread (So as not to block the main UI), but this triggers a multithreading issue

https://bugzilla.xamarin.com/28/28047/bug.html github.com/ ArduPilot/ MissionPlanner/issues/208