Description of problem:
gnome builder close the window and close the process when trying to access to a folder that doesn't exit

Version-Release number of selected component (if applicable):
Builder - Version 3.16.3

How reproducible:
always

Steps to Reproduce:
1. Launch gnome builder
2. create a project in a folder
3. close builder
4. move the folder to an other location
5. launch builder
6. select the project

Actual results:

gnome builder close the window and close the process
[jean-baptiste@jportable osm-modifications]$ gnome-builder 
23:01:37.2068 gb-greeter-window[9409]:  WARNING: No implementations of extension point.

Expected results:
Maybee an error message, but not a crash