fix(developer): address crashes in touch layout editor

[mcdurdin]

Crashes reported in user testing on #16520.

• Fix crash selecting layer after deleting a layer (from fix(developer): remember presentation and layer when switching platforms in touch layout editor #16509)
• Fix crash resetting 'next layer' for flicks after layer deletion (long-standing issue)

See-also: #16509
See-also: #16520
Cherry-picked-from: 099d039

User Testing

TEST_DELETE_LAYER: In the touch layout editor, press F12 to open the Developre Console. Add a key with a flick and set the flick's 'next layer' to Shift. Then delete the Shift layer. Verify that no crashes are reported.

.

[keymanapp-test-bot]

User Test Results

Test specification and instructions

• ✅ TEST_DELETE_LAYER (PASSED) (notes)

Test Artifacts

• Developer
  • Keyman Developer
  • Compiler Regression Tests
  • kmcomp.zip
• Keyboards
  • Test Keyboards

[rc-swag]

lgtm

[Meng-Heng]

Test Results

• TEST_DELETE_LAYER (PASSED):

1. Launch Keyman Developer 19.0.289-alpha-test-16572
2. In the touch layout editor
3. Press F12 to open the Developer Console
4. Add a flick to a key and set the flick's 'next layer' to Shift
5. Delete the Shift layer
6. VERIFIED: No crashes reported. The flick's next layer return to (none)

[keyman-server]

Changes in this pull request will be available for download in Keyman version 19.0.291-alpha