Fix: Exception in tree view code in new case

This isn't a problem in 3.0 or master, but I'm porting the spreadsheet
data set region to a tree view and ran into this. This line needs to
whether the function is empty before calling it.

Differential Revision: https://developer.blender.org/D13197

source/blender/editors/interface/tree_view.cc

@@ -546,7 +546,7 @@ uiButTreeRow *AbstractTreeViewItem::tree_row_button()
 
 void AbstractTreeViewItem::change_state_delayed()
 {
-  if (is_active_fn_()) {
+  if (is_active_fn_ && is_active_fn_()) {
     activate();
   }
 }