Fix bugs that cause crashes on layers copy/paste/delete.

Fix bugs that cause crashes on layers copy/paste/delete.

The bug can be reproduced consistently by following steps:

- start viking, create a map layer
- select the map layer
- click on copy
- select some text on a terminal
- right click on the selected text.

At this stage, error messages appears such as:
(viking:5219): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `VikLayer'

(viking:5219): Gdk-WARNING **: Error converting from UTF-8 to STRING: Invalid byte sequence in conversion input

The memory is corrupted by then, other operations will crash viking eventually.

Signed-off-by: Quy Tonthat <qtonthat@gmail.com>