Prune navigation history when assemblies are removed

Ports the WPF assemblyList_CollectionChanged history-prune from
ILSpy/AssemblyTree/AssemblyTreeModel.cs to the Avalonia
DockWorkspace.OnAssemblyListChanged handler. NavigationHistory<T>
already exposed the RemoveAll(Predicate<T>) primitive; the caller
wiring was the missing piece. Without it, a tree-row click after
removing an assembly walked through NavigationEntry.DisplayText
on a stale TreeNodeEntry, which hit MemberReferenceTreeNode.Signature
-> Language.EntityToString -> ILAmbience.ConvertSymbol and NRE'd on
a now-null ParentModule.

Assisted-by: Claude:claude-opus-4-7:Claude Code

ICSharpCode.Decompiler/IL/ILAmbience.cs

@@ -53,11 +53,23 @@ namespace ICSharpCode.Decompiler.IL
 
 		void ConvertSymbol(StringWriter writer, ISymbol symbol)
 		{
-			var metadata = (symbol as IEntity)?.ParentModule!.MetadataFile?.Metadata;
-			var token = (symbol as IEntity)?.MetadataToken ?? default;
+			var entity = symbol as IEntity;
+			var metadata = entity?.ParentModule?.MetadataFile?.Metadata;
+			var token = entity?.MetadataToken ?? default;
 
 			var output = new PlainTextOutput(writer);
 
+			// When the symbol's owning module has been torn down (stale entity pinned in
+			// navigation history after the assembly was unloaded/reloaded), the rest of
+			// the switch can't read its metadata reader. Fall back to the symbol's name
+			// so callers still get sensible display text — the NRE would otherwise
+			// surface up through every consumer that formats a stale history entry.
+			if (metadata == null)
+			{
+				writer.Write(symbol?.Name ?? string.Empty);
+				return;
+			}
+
 			switch (symbol)
 			{
 				case IField f:

ILSpy/Docking/DockWorkspace.cs

@@ -183,12 +183,24 @@ namespace ILSpy.Docking
 		void OnAssemblyListChanged(object? sender, ILSpy.Util.CurrentAssemblyListChangedEventArgs e)
 		{
 			var inner = e.Inner;
+
+			// On Reset (assembly list wholesale-cleared), drop ALL history — every entry is
+			// stale by definition. Mirrors WPF's assemblyList_CollectionChanged.
+			if (inner.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Reset)
+			{
+				PruneHistoryAfterAssemblyListChange(removed: null);
+				return;
+			}
+
 			if (inner.OldItems is not { Count: > 0 } oldItems)
 				return;
 			var removed = new HashSet<ICSharpCode.ILSpyX.LoadedAssembly>(
 				oldItems.OfType<ICSharpCode.ILSpyX.LoadedAssembly>());
 			if (removed.Count == 0)
 				return;
+
+			PruneHistoryAfterAssemblyListChange(removed);
+
 			if (factory.Documents?.VisibleDockables is not { } visible)
 				return;
 
@@ -212,14 +224,29 @@ namespace ILSpy.Docking
 				if (!anyAlive)
 					factory.CloseDockable(dockable);
 			}
+		}
 
-			// TODO: layout persistence (load on startup, save on exit). DockSerializer.SystemTextJson
-			// trips System.Text.Json's MaxDepth=64 limit on our layout because Dock's JsonConverterList<T>
-			// calls JsonSerializer.Serialize per list element, which doesn't preserve the writer's
-			// reference-tracking state between calls — so Owner/Factory back-refs aren't deduplicated as
-			// $ref. Options when revisiting: try Dock.Serializer.Newtonsoft (better cycle handling),
-			// or build a custom JsonSerializerOptions with MaxDepth raised + replicate Dock's internal
-			// polymorphic resolver.
+		// Drops history entries whose tree node lives inside a now-removed assembly. With
+		// <paramref name="removed"/> == null, drops everything (used for the Reset action).
+		// Without this, Back/Forward would surface entries whose Node reference has been
+		// detached from the live tree, and formatting their DisplayText routes through
+		// language-specific formatters that NRE on a stale IEntity.ParentModule.
+		void PruneHistoryAfterAssemblyListChange(HashSet<ICSharpCode.ILSpyX.LoadedAssembly>? removed)
+		{
+			if (removed == null)
+			{
+				history.RemoveAll(_ => true);
+			}
+			else
+			{
+				history.RemoveAll(entry =>
+					entry is Navigation.TreeNodeEntry t
+					&& t.Node.AncestorsAndSelf()
+						.OfType<TreeNodes.AssemblyTreeNode>()
+						.Any(a => removed.Contains(a.LoadedAssembly)));
+			}
+			NavigateBackCommand.NotifyCanExecuteChanged();
+			NavigateForwardCommand.NotifyCanExecuteChanged();
 		}
 
 		void OnDocumentMembershipChanged(object? sender, EventArgs e) => UpdateLastDocumentCanClose();