[gnome-games] core-source: Don't use core descriptor before null checking it

[Alexander Mikhaylenko <alexm src gnome org>]

commit 7ffc2aaa7995a0db87df2d00720eda645497e0b2
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Tue Mar 16 23:07:21 2021 +0500

    core-source: Don't use core descriptor before null checking it
    
    Fix a critical.

 src/retro/retro-core-source.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/retro/retro-core-source.vala b/src/retro/retro-core-source.vala
index b6b6c0b8..8fe8af81 100644
--- a/src/retro/retro-core-source.vala
+++ b/src/retro/retro-core-source.vala
@@ -37,7 +37,6 @@ public class Games.RetroCoreSource : Object {
 
                var platform_id = platform.get_id ();
                var firmware_manager = new FirmwareManager ();
-               var core = new RetroCore (core_descriptor);
 
                if (core_descriptor == null) {
                        var mime_types = platform.get_mime_types ();
@@ -45,6 +44,8 @@ public class Games.RetroCoreSource : Object {
                        throw new RetroError.MODULE_NOT_FOUND ("No module found for platform “%s” and MIME 
types [ “%s” ].", platform_id, string.joinv ("”, “", mime_types));
                }
 
+               var core = new RetroCore (core_descriptor);
+
                firmware_manager.is_all_firmware_valid (core, platform);
        }