fix(developer): handle window already destroyed when closing editor

[mcdurdin]

Fixes: #15617
Fixes: KEYMAN-DEVELOPER-1JD
Fixes: KEYMAN-DEVELOPER-36K
Fixes: KEYMAN-DEVELOPER-2P5
Test-bot: skip

[keymanapp-test-bot]

User Test Results

Test specification and instructions

User tests are not required

Test Artifacts

• Windows
  • Keyman for Windows - build : all tests passed (no artifacts on BuildLevel "build")
  • FirstVoices Keyboards for Windows - build : all tests passed (no artifacts on BuildLevel "build")
  • FirstVoices Keyboards for Windows (old PRs) - build : all tests passed (no artifacts on BuildLevel "build")
  • Keyman for Windows (old PRs) - build : all tests passed (no artifacts on BuildLevel "build")
  • Text Editor (ARM64) - build : all tests passed (no artifacts on BuildLevel "build")
  • Text Editor (x64) - build : all tests passed (no artifacts on BuildLevel "build")
  • Text Editor (x86) - build : all tests passed (no artifacts on BuildLevel "build")