Thanks all; wow, that's a sneaky case to clear the root during the recursion to handle(FL_FOCUS).

Doing that second test for _root == NULL shows how this problem can crop up anywhere, so perhaps doing anything directly with _root is inherently dangerous.

Changing _root->find_clicked() to just find_clicked() fixes the problem too, as it does the same operation and includes the check for (_root == NULL) in the method, so that's a cleaner solution, e.g.

-     Fl_Tree_Item *item = _root->find_clicked(_prefs, 0);
+     Fl_Tree_Item *item = find_clicked(0);      // #971

And in fact that pattern should be applied throughout the handle() method wherever _root->find_clicked() is used. I'll try applying that later today; I'm not quite awake yet.

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