Kolourpaint: x seconds Freeze on save and every keystroke

[DarkTrick]

Consequence of this bug: Unable to (properly) use save function of Kolourpaint

Steps to reproduce:
====================
- Open Kolourpaint
- Press Ctrl+S for saving it
- Dialog opens
- Expected: I can type in a name and save
- Actual: Kolourpaint freezes for many seconds (10? 30?). Then I can press one or two letters, that are shown, then Kolourpaint freezes again for many seconds.

Workaround:
- Save the file in the default location under a random name (e.g. "a"), use the default image format.
- Rename and convert in another program.

lsb_release -rd
--------------------
Description: Ubuntu 20.10
Release: 20.10

apt-cache policy kolourpaint
----------------------------
kolourpaint:
  Installed: 4:20.08.1-0ubuntu1
  Candidate: 4:20.08.1-0ubuntu1
  Version table:
 *** 4:20.08.1-0ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu groovy/universe amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: kolourpaint 4:20.08.1-0ubuntu1
ProcVersionSignature: Ubuntu 5.8.0-45.51-generic 5.8.18
Uname: Linux 5.8.0-45-generic x86_64
ApportVersion: 2.20.11-0ubuntu50.5
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: XFCE
Date: Wed Mar 17 15:47:52 2021
SourcePackage: kolourpaint
UpgradeStatus: No upgrade log present (probably fresh install)