Test/crash in MonoTouchFixtures.UIKit.DocumentTest

[rolfbjarne]

The MonoTouchFixtures.UIKit.DocumentTest test sometimes fails with:

MonoTouchFixtures.UIKit.DocumentTest
    [PASS] Fields
    [PASS] PerformAsynchronousFileAccess_Null
    [FAIL] Save : ObjCRuntime.RuntimeException : Failed to marshal the Objective-C object 0x6000014e0b40 (type: MonoTouchFixtures_UIKit_DocumentTest_MyDocument). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'MonoTouchFixtures.UIKit.DocumentTest+MyDocument' does not have a constructor that takes one NativeHandle argument).
        Additional information:
        	Selector: contentsForType:error:
        	Method: MonoTouchFixtures.UIKit.DocumentTest/MyDocument:ContentsForType (string,Foundation.NSError&)
        		   at ObjCRuntime.Runtime.RethrowManagedException(IntPtr )
        		   at ObjCRuntime.Runtime.rethrow_managed_exception(IntPtr original_exception_gchandle, IntPtr* exception_gchandle)
        		   at UIKit.UIDocument.Save(NSUrl , UIDocumentSaveOperation , UIOperationHandler )
        		   at MonoTouchFixtures.UIKit.DocumentTest.Save()
        		   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
        		   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object , BindingFlags )
MonoTouchFixtures.UIKit.DocumentTest : 3.8699 ms

Full log:

execute-MacCatalyst-20260212_062437.txt

Othertimes it crashes (I'll get a crash report next time).

This was first reported in maccore: https://github.com/xamarin/maccore/issues/2283

A previous attempt at fixing this didn't work: #24165