clicking "Addin Manager" causes unhandled exception on Pinta 1.8

[Rega Fox]

I have no addins and this is from flathub beta channel.

When i click Addins Manager, i get the following message:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.NotImplementedException: The method or operation is not implemented.
   at Pinta.Actions.AddinManagerAction.Activated(Object sender, EventArgs e)
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at GLib.Signal.ClosureInvokedCB(Object o, ClosureInvokedArgs args)
   at GLib.SignalClosure.MarshalCallback(IntPtr raw_closure, IntPtr return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr invocation_hint, IntPtr marshal_data)

[Cameron White (cameronwhite91)]

This is a known issue, assuming that the flathub beta channel is using the GTK3 branch for Pinta 1.8. The add-in manager (and Mono.Addins library) haven't been ported to GTK3 yet.

[Cameron White (cameronwhite91)]

Addressed by https://github.com/PintaProject/Pinta/commit/7fa71e0c0eb48ce7d630d05f0929d3833d00a769
Add-in support will remain disabled for Pinta 1.8 so that we can actually release it :) The Mono.Addins library needs to be updated to .NET 6 / GTK 3