Geary crashes when selecting a email provider

[Maik]

When opening Geary and trying to select any of the provided email prodivers, it just crashes.
Running Geary from the terminal and doing the same thing shows this:

*[wrn] 06:21:00.0488 Gtk:Theme parsing error: gtk.css:5480:34: Invalid name of pseudo-class
*[wrn] 06:21:03.0758 Gtk:gtk_window_set_titlebar() called on a realized window

This is happening on both Ubuntu 20.04.1 and 20.10, running Unity7.
---
ProblemType: Bug
ApportVersion: 2.20.11-0ubuntu27.9
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: Unity:Unity7:ubuntu
DistroRelease: Ubuntu 20.04
InstallationDate: Installed on 2020-08-16 (61 days ago)
InstallationMedia: Ubuntu Unity 20.04.1 "Focal Fossa" (20200814)
Package: geary 3.36.1-1
PackageArchitecture: amd64
ProcVersionSignature: Ubuntu 5.4.0-51.56-generic 5.4.65
Tags: focal
Uname: Linux 5.4.0-51-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin lxd plugdev sambashare sudo
_MarkForUpload: True

[Maik (maik-adamietz)]

apport information

[Maik (maik-adamietz)]

apport information

[Maik (maik-adamietz)]

apport information

[Launchpad Janitor (janitor)]

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

[lotuspsychje (lotuspsychje)]

Ubuntu-desktop 20.04.1 all up to date to latest @ 16/10/2020 with kernel 5.4.0-51-generic

Installed Unity desktop from the repos, launched geary, on 'other accounts'
the program crashes with error: *[wrn] 15:28:18.0995 Gtk:gtk_window_set_titlebar() called on a realized window

this does not happen on ubuntu-desktop on gnome

[Rudra Saraswat (rs2009)]

I've added that this issue only happens in Unity7 in the bug description.

[Rudra Saraswat (rs2009)]

gtk3-nocsd seems to be causing the issue.