Gdk BadMatch with menus

I got a serious problem with GTK, and I have no clue how to handle it.
The problem is about menus.

When changing the nodes in the tree, the context menu and the Edit menu
will be changed; the New menu entry will reflect the current node's
abilities. My first implementation destroyed the "New Object" menu item
including its submenu, and appended it freshly according to the node.
This lead this message on every node change:
tk-WARNING **: gtk_menu_attach_to_widget(): menu already attached to
GtkMenuItem
Gtk-CRITICAL **: file gtkwidget.c: line 1388 (gtk_widget_destroy):
assertion `GTK_IS_WIDGET (widget)' failed.

Then I changed the coding. Now, I only delete all menu items from the
"New Object" submenu, and append new items as appropriate. The mentioned
debug messages disappeared.
Still, the application will end with the message

Gdk-ERROR **: BadMatch (invalid parameter attributes)
serial 12530 error_code 8 request_code 59 minor_code 0
Program exited with code 01

when using the context menu (normal menu seems ok). To reproduce this,
select "New Object -> Index" on a table.
Unfortunately, the app simply exits, so the debugger is useless.

I didn't get useful hints from internet search on the topid "BadMatch".

Any clues? GTK gurus around?

Regards,
Andreas