[de.mardukcorp.kawaiinihongo] Hangs on loading flash cards.

Affected app
Name: kawaiiNihongo: Learn Japanese
Package id: de.mardukcorp.kawaiinihongo

Describe the bug
The freemium app contains a flash card section for teaching which doesn't work. Attempting to load it causes the app to hang on the loading screen indefinitely.

To Reproduce
Steps to reproduce the behavior:

1. Open the app and go to the "School" section. (For first time users, they will be taken to step 3 automatically after the intro sequence.)
2. Click on 'Learn'
3. Click on any of the available sections. (Hiragana 1 is the only one available at the start and is also free.)
4. Click Start Lesson.
5. The app will hang on the preparing lesson screen.

Expected behavior
A clear and concise description of what you expected to happen.
The lesson should start and the first flash card should be displayed.

System
Android Version: 10
Custom ROM: ASOP-based GSI from https://github.com/phhusson/treble_experimentations

microG
microG Core version: 0.2.22.212658
microG Self-Check results: All ticked

Additional context
Uses Firebase Messaging.
Seems to be trying to handle a caught exception at the time of the hang, but the handler isn't available due to it not being implemented. (See below.) If it was, we might be able to see what caused the hang.

Logcat recorded this at the time of the hang:
[04-15 02:36:44.461 6303:6402 W/Unity]
Multiple UIParticleOverlayCamera in scene.
#0 0xc066161d (libunity.so) ? 0x0
#1 0xc053d73f (libunity.so) ? 0x0
#2 0xc0235f3d (libunity.so) ? 0x0
#3 0xc0235eab (libunity.so) ? 0x0
#4 0xb9bea30c (libil2cpp.so) ? 0x0
#5 0xb9be095c (libil2cpp.so) ? 0x0
#6 0xba64afcc (libil2cpp.so) ? 0x0
#7 0xb82f01b8 (libil2cpp.so) ? 0x0
#8 0xb84eaae4 (libil2cpp.so) ? 0x0
#9 0xc01cd7f9 (libunity.so) ? 0x0
#10 0xc01d6d2f (libunity.so) ? 0x0
#11 0xc01d7ff9 (libunity.so) ? 0x0
#12 0xc01fdc7b (libunity.so) ? 0x0
#13 0xc01dde5b (libunity.so) ? 0x0
#14 0xc01de0fd (libunity.so) ? 0x0
#15 0xc01dde05 (libunity.so) ? 0x0
#16 0xc0208589 (libunity.so) ? 0x0
#17 0xc02084cd (libunity.so) ? 0x0
#18 0xc0208475 (libunity.so) ? 0x0
#19 0xc0171633 (libunity.so) ? 0x0
#20 0xc01717a5 (libunity.so) ? 0x0
#21 0xc01713fd (libunity.so) ? 0x0
#22 0xc017124b (libunity.so) ? 0x0
#23 0xc0171cef (libunity.so) ? 0x0
#24 0xc01720c9 (libunity.so) ? 0x0
#25 0xc0172167 (libunity.so) ? 0x0
#26 0xc0169dad (libunity.so) ? 0x0
#27 0xc0169d

[04-15 02:36:44.463 6303:6402 D/CubismCore]
Live2D Cubism SDK Core Version 3.3.1

[04-15 02:36:44.465 6303:6402 D/CubismCore]
Live2D Cubism SDK Core Version 3.3.1

[04-15 02:36:44.829 6303:6402 E/Unity]
NullReferenceException: Object reference not set to an instance of an object.
at Firebase.Crashlytics.Crashlytics.LogException (System.Exception exception) [0x00000] in <00000000000000000000000000000000>:0
at LearnLessonScene.handleStartLessonException (System.Exception e) [0x00000] in <00000000000000000000000000000000>:0
at LearnLessonScene.Start () [0x00000] in <00000000000000000000000000000000>:0

The log names LearnLessonScene.Start, LearnLessonScene.handleStartLessonException, and Firebase.Crashlytics.Crashlytics.LogException. Begin by reproducing the lesson-loading hang and tracing the logged exception through the relevant Android or microG integration. Done means the free Hiragana 1 lesson reaches its first flash card instead of remaining on the preparing screen.