#89 External Plugin creation re-done display error

[Louis Carrese]

When undoing then redoing the creation of an external plugin, the plugin is not displayed in the Rack Panel.

Steps to reproduce :
Create an external plugin (ie: any LADSPA)
Delete the rack
Undo the deletion

Obtained result :
The plugin is not displayed

Expected result :
The plugin should be correctly displayed

Additional notes :
It's only a problem of display. If the session is saved and reloaded, the plugin is still there.