Fix null reference errors - currently working according to my overlord

- Fixed updateUndoButton null reference at line 1543
- Added null check before accessing DOM element properties
- Tracks now load properly
- Hamburger menu confirmed functional
- All critical JavaScript execution errors resolved

This commit represents a stable, working state after multiple fix attempts documented in FIX_ATTEMPTS.md

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

index.html

@@ -1540,9 +1540,11 @@
 
         function updateUndoButton() {
             const btn = document.getElementById('undoBtn');
-            btn.textContent = undoStack.length > 0 ? `Undo (${undoStack.length})` : 'Undo';
-            btn.disabled = undoStack.length === 0;
-            btn.style.opacity = undoStack.length === 0 ? '0.5' : '1';
+            if (btn) {
+                btn.textContent = undoStack.length > 0 ? `Undo (${undoStack.length})` : 'Undo';
+                btn.disabled = undoStack.length === 0;
+                btn.style.opacity = undoStack.length === 0 ? '0.5' : '1';
+            }
         }
 
         // GPS functions