Fixed a crash in Switcher

I'm not sure exactly what caused this that other simpler cases were not,
but essentially nodes were already removed once by the time this loop is
evaluated, so we can skip adding them back to the list again.

CHANGELOG.md

@@ -9,6 +9,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+- Fixed a panic that could occur when removing some deeply nested hierarchies
+  from a window.
+
 ### Added
 
 - `AnimationRecorder::animate_keypress` is a new helper that animates a single

src/tree.rs

@@ -506,7 +506,7 @@ impl TreeData {
             removed_node
                 .children
                 .into_iter()
-                .map(|id| self.nodes[id].widget.id()),
+                .filter_map(|id| self.nodes.get(id).map(|node| node.widget.id())),
         );
 
         if let Some(next_focus) = removed_node.widget.next_focus() {