Can't add external tool directly

1. Select "Tools" -> "External" -> "Configure" from the menu.
2. Click "Add" and select "Add Tool" from the drop-down menu.
   Nothing will happen at the GUI. Creator will print

   edit: index was invalid
   edit: editing failed
   

   to the command line.

Instead, a reasonable default value should be set so that the user can add a tool directly. Additionally the menu should be grayed if an action is impossible.