Canceling training run while running freezes app

Investiate + fix canceling a training run through the cancel button on the progress bar- this currently freezes the app.

Originally reported for a test run on Mac, now exists on test run on Windows OS / EC2 instance `winsegml2`

### Step to reproduce:
1. Start training
2. Once the progress modal shows up, either wait for a bit (let the file cached, or let the first few epochs run) or without waiting, click the [Cancel] button below the progress bar in the modal
3. The progress modal would close, napari & the plugin look like they've returned to normal >> this part makes it look like "cancelling" works
4. Click anywhere on napari, the napari title bar would change to "Not responding"
5. Wait for a long time and the "not responding" is not resolving