Codenav plugin: makes geany crash when adding new file types

[Zizilla-dev]

Geany crashes when adding new file type to the Code Navigator plugin.

1.) install Code navigator plugin, and select it in the plugins list
2.) go to Preferences
3.) press Add
4.) add new extension
5.) press Apply

Result: Geany quits immediately

It is reproducible my machine; Arch linux, all packages updated, Budgie desktop.

mod: The issue only reproducible, if the left column is filled and the header file part is left empty.
So this is not a big issue, only a missing error handling.