xsane does not react

[Matthias Homann]

when i select a pagesize in the preview window or when i select a type in the main window
xsane does not react and fills the console with lines like
```
(xsane:92335): Gtk-CRITICAL **: 18:32:35.142: IA__gtk_widget_event: assertion 'WIDGET_REALIZED_FOR_EVENT (widget, event)' failed

(xsane:92335): Gtk-CRITICAL **: 18:32:35.154: IA__gtk_widget_event: assertion 'WIDGET_REALIZED_FOR_EVENT (widget, event)' failed

(xsane:92335): Gtk-CRITICAL **: 18:32:35.164: IA__gtk_widget_event: assertion 'WIDGET_REALIZED_FOR_EVENT (widget, event)' failed

(xsane:92335): Gtk-CRITICAL **: 18:32:35.174: IA__gtk_widget_event: assertion 'WIDGET_REALIZED_FOR_EVENT (widget, event)' failed

(xsane:92335): Gtk-CRITICAL **: 18:32:35.174: IA__gtk_widget_event: assertion 'WIDGET_REALIZED_FOR_EVENT (widget, event)' failed

(xsane:92335): Gtk-CRITICAL **: 18:32:35.184: IA__gtk_widget_event: assertion 'WIDGET_REALIZED_FOR_EVENT (widget, event)' failed

```

uname -a
Linux wahalla 5.8.0-45-generic #51~20.04.1-Ubuntu SMP Tue Feb 23 13:46:31 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

apt list | grep xsane

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

xsane-common/focal-updates,focal-updates,now 0.999-8ubuntu2.1 all [Installiert,automatisch]
xsane/focal-updates,now 0.999-8ubuntu2.1 amd64 [installiert]

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: xsane 0.999-8ubuntu2.1
ProcVersionSignature: Ubuntu 5.8.0-45.51~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-45-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Mon Mar 29 18:12:10 2021
EcryptfsInUse: Yes
InstallationDate: Installed on 2020-11-03 (145 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
SourcePackage: xsane
UpgradeStatus: No upgrade log present (probably fresh install)