Attempting to add any widget causes the app to crash. It doesn't seem to matter if I drag the widget or use the keyboard (I'm an orca screen reader user).

This is the output to stderr:

(glade:3873): Gtk-WARNING **: 19:06:46.590: Theme parsing error: gtk-widgets.css:3135:13: 'max-width' is not a valid property name

(glade:3873): Gtk-WARNING **: 19:06:46.590: Theme parsing error: gtk-widgets.css:3136:14: 'max-height' is not a valid property name
GladeUI-Message: 19:06:46.812: 1 missing displayable value for GtkWidget::events
GladeUI-Message: 19:06:46.813: 1 missing displayable value for GtkWidget::AtkObject::accessible-role
GladeUI-Message: 19:06:46.915: 2 missing displayable value for GtkEntry::input-hints
GladeUI-Message: 19:06:47.066: No displayable values for property GtkMenu::anchor-hints
GladeUI-Message: 19:06:47.123: 2 missing displayable value for GtkAboutDialog::license-type
GladeUI-Message: 19:06:47.235: 1 missing displayable value for GtkCellRendererAccel::accel-mode
GladeUI-Message: 19:06:47.353: No displayable values for property WebKitSettings::hardware-acceleration-policy
GladeUI-Message: 19:06:47.353: Glade needs artwork; a default icon will be used for the following classes:
 WebKitSettings needs an icon named 'widget-webkit2gtk-settings'
 GThemedIcon needs an icon named 'widget-gtk-themedicon'
 WebKitWebView needs an icon named 'widget-webkit2gtk-webview'
 GtkApplicationWindow needs an icon named 'widget-gtk-applicationwindow'

(glade:3873): Gdk-WARNING **: 19:07:23.251: ../../../../../gdk/x11/gdkwindow-x11.c:5576 drawable is not a native X11 window

(glade:3873): Gdk-ERROR **: 19:07:23.252: The program 'glade' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadWindow (invalid Window parameter)'.
  (Details: serial 348072 error_code 3 request_code 20 (core protocol) minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: glade 3.22.1-2
ProcVersionSignature: Ubuntu 4.18.0-11.12-generic 4.18.12
Uname: Linux 4.18.0-11-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
CurrentDesktop: MATE
Date: Sat Dec 8 19:30:07 2018
InstallationDate: Installed on 2018-11-23 (14 days ago)
InstallationMedia: Ubuntu-MATE 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.2)
SourcePackage: glade
UpgradeStatus: No upgrade log present (probably fresh install)