I think it was the KB4601887 update (Cumulative Update for .NET Framework) that broke things as acknowledged in the Microsoft notes for it.. It mentions that crashes can happen "when disposing an HwndSource whose RootVisual is null,
a situation that arises in Visual Studio when docking or splitting windows, and could arise in other apps."
Workaround
This issue was resolved in KB4601558 for .NET Framework 4.7.2, and KB4601555 for .NET Framework 4.8.
I'm seeing KB4602298 (Cumulative Update Preview for .NET Framework) listed as an update on my Windows 10 PC which seems to incorporate these two fixes. Installing it resolved my issues with the PowerPMAC IDE crashes.
More details here: https://www.bleepingcomputer.com/news/microsoft/recent-windows-10-updates-cause-visual-studio-wpf-app-crashes/