(mate-font-viewer:20507): Gtk-CRITICAL **: 18:53:19.336: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed

[Norbert]

Steps to reproduce:
1. Install Ubuntu MATE 20.04 LTS
2. Launch `mate-font-viewer` from the terminal
3. Wait `mate-font-viewer` to load all fonts
4. Click *Search* button and start to enter font name

Expected results:
* font(s) is/are found, the terminal is empty

Actual results:
* font(s) is/are found, but terminal has Gtk critical messages like

```
(mate-font-viewer:21717): Gtk-CRITICAL **: 18:57:01.681: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed

```

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: mate-control-center 1.24.0-0ubuntu1
ProcVersionSignature: Ubuntu 5.4.0-14.17-generic 5.4.18
Uname: Linux 5.4.0-14-generic x86_64
ApportVersion: 2.20.11-0ubuntu16
Architecture: amd64
CurrentDesktop: MATE
Date: Sat Feb 15 18:53:36 2020
InstallationDate: Installed on 2020-02-15 (0 days ago)
InstallationMedia: Ubuntu-MATE 20.04 LTS "Focal Fossa" - Alpha amd64 (20200215)
SourcePackage: mate-control-center
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Norbert (nrbrtx)]

Bug still exists in latest Ubuntu MATE 22.04 LTS.