backgroundApps: Don't crash if app id can't be looked up
I'm looking at a log file with this trace back:
JS ERROR: TypeError: a.app is null
_sync/<@resource:///org/gnome/shell/ui/status/backgroundApps.js:226:17
_sync@resource:///org/gnome/shell/ui/status/backgroundApps.js:225:14
_init/</<@resource:///org/gnome/shell/ui/status/backgroundApps.js:170:67
@resource:///org/gnome/shell/ui/init.js:21:20
This commit should avoid the failure by filtering out items where lookup_app fails.