Avoid using already-destroyed parts of wxChoice/wxComboBox during destruction

When used as a wxDVC cell editor, GtkComboBox has already destroyed
it's model and child GtkEntry by the time our dtor is called.
See #17034

src/gtk/combobox.cpp

@@ -99,7 +99,10 @@ wxEND_EVENT_TABLE()
 wxComboBox::~wxComboBox()
 {
     if (m_entry)
+    {
         GTKDisconnect(m_entry);
+        g_object_remove_weak_pointer(G_OBJECT(m_entry), (void**)&m_entry);
+    }
 }
 
 void wxComboBox::Init()
@@ -213,6 +216,7 @@ void wxComboBox::GTKCreateComboBoxWidget()
     g_object_ref(m_widget);
 
     m_entry = GTK_ENTRY(gtk_bin_get_child(GTK_BIN(m_widget)));
+    g_object_add_weak_pointer(G_OBJECT(m_entry), (void**)&m_entry);
 }
 
 GtkEditable *wxComboBox::GetEditable() const