Can't open the "Manage Plug-ins" button in MacOS Mail

MacOSX version: catalina 10.15.4
Following the installation instructions I installed the Mail Plug-in.

First, running AltServer:

Notification Connection Error: Error Domain=com.rileytestut.AltServer Code=1 "Could not connect to AltServer."

Next, installing the Mail Plug-in:

Downloaded plugin to URL: file:///var/folders/kh/l4pck1457qq4x1fqp1k7m55m0000gn/T/CFNetworkDownload_QBwOW6.tmp
Running program: /bin/mkdir
Exit code: 0
Running program: /usr/bin/unzip
Exit code: 0
Running program: /usr/bin/defaults
Exit code: 0
Finished installing Mail plug-in!

Mail app is closed. When opening Mail again > Preferences > General, attempting to click on the "Manage Plug-ins…" button, the Plug-ins window won't open and the following error occurs:

Assertion failure in -[GeneralPreferences presentViewController:animator:], /AppleInternal/BuildRoot/Library/Caches/com.apple.xbs/Sources/AppKit/AppKit-1894.40.150/Controllers/NSViewController.m:849

After uninstalling the AltServer Mail plug-in, the Mail plug-in window opens again.

Start by reproducing the failure on macOS Catalina with the AltServer Mail plug-in installed, then compare Mail's behavior before and after installation. Investigate the plug-in and its interaction with Mail's Preferences window; done means the Manage Plug-ins window opens normally while the plug-in remains installed.