Initial Install doesn't create icons

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
After installing a flatpak from flathub, the package's icon does not show up in the applications-menu, and the plank icon is a gray gear (missing icon).

Restarting both wingpanel and plank resolves the issue (so does restarting the computer), and from that point forward, the icon is available in both the applications-menu and in plank.

### To Reproduce
Steps to reproduce the behavior:
1. Go to flathub.org and select a package. In my case, I selected slack.
2. Click on the `Install` button and open the flatpakref.
3. Select "I understand" in sideload and click `Install`.
4. After the installation is complete, click `Open App`.
5. The icon in plank will be the gear icon, and if you click on `Applications` on the top left, slack is not there.

Next you can restart wingpanel/plank (or just restart your pc), and the icon should be available.

### Expected behavior
I would expect the new application's icon to show up in applications-menu and plank immediately, instead of after a restart.

### Screenshots or screen recordings
I'll get some screenshots in a little bit. coming soon!

### Logs

same as screenshots, I'll try to gather some logs after running through the steps oncemore.

## Platform Information

- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version