Fixing an edge case in computing tab order

Removing widgets would affect the visual order, and sometimes nodes just
aren't available anymore. It might be that this should be cleaned up
more aggressively during widget removal, but by ignoring the removed
nodes, the code just becomes more resilliant to similar edge cases in
the future.

src/tree.rs

@@ -181,7 +181,7 @@ impl Tree {
 
             let mut index = 0;
             while index < unordered.len() {
-                let layout = match &data.nodes[unordered[index]].layout {
+                let layout = match data.nodes.get(unordered[index]).and_then(|n| n.layout) {
                     Some(layout) if layout.size.width > 0 && layout.size.height > 0 => layout,
                     _ => {
                         unordered.remove(index);