fix Undo; crashes due to race in Gtk

Some Redraws occur during a model clear when the row changed signal
is emmitted.  Model is only partially cleared at that point.
Under some conditions, some more of the model gets
cleared before the redraw completes, and redraw fails.
fixes #9932

gramps/gui/undohistory.py

@@ -120,10 +120,11 @@ def __init__(self, dbstate, uistate):
         scrolled_window.add(self.tree)
         self.window.vbox.pack_start(scrolled_window, True, True, 0)
 
+        self.sel_chng_hndlr = self.selection.connect('changed',
+                                                     self._selection_changed)
         self._build_model()
         self._update_ui()
 
-        self.selection.connect('changed', self._selection_changed)
         self.show()
 
     def _selection_changed(self, obj):
@@ -226,6 +227,7 @@ def _update_ui(self):
             )
 
     def _build_model(self):
+        self.selection.handler_block(self.sel_chng_hndlr)
         self.model.clear()
         fg = bg = None
 
@@ -243,6 +245,7 @@ def _build_model(self):
             mod_text = txn.get_description()
             self.model.append(row=[time_text, mod_text, fg, bg])
         path = (self.undodb.undo_count,)
+        self.selection.handler_unblock(self.sel_chng_hndlr)
         self.selection.select_path(path)
 
     def update(self):