Importing .mcpack, .mcaddon, or .mcworld files while the game is opened no longer works

If you try to import an .mcpack, .mcaddon, or .mcworld files while the game is opened, only the game window will be focused, the pack/world will not be imported to the game.

Steps to reproduce:

1. Download the attached .mcpack files in the report, or download any .mcaddon or .mcworld files from 3rd party sites such as MCPEDL or Modbay

2. Launch Minecraft Preview and wait until the Main menu appears

3. Double-click the files you've downloaded, and select Minecraft Preview for Windows if asked

4. Observe

Expected results:
The game window will be focused, then a notification appears at the top of the screen, and the pack with the name "copper_lantern_anim_fix" is available on "My Packs".

Observed results:
The game window will be focused, but there's no notification at all at the top of the screen, and the pack with the name "copper_lantern_anim_fix" is not available on "My Packs".