Add missing build_tree() abstract method implementation

- Implement build_tree() method required by NavigationView
- Calls goto_handle() with active family to rebuild display
- Fixes TypeError when loading MyTimeline view

MyTimeline.py

@@ -187,6 +187,12 @@ class MyTimelineView(NavigationView):
         self.scrolledwindow.add(self.drawing_area)
         return self.scrolledwindow
 
+    def build_tree(self):
+        """Rebuilds the current display. Called when the view becomes visible."""
+        active_handle = self.get_active()
+        if active_handle:
+            self.goto_handle(active_handle)
+
     def goto_handle(self, handle):
         """Called when the active family changes."""
         if handle == self.active_family_handle: