[Bug] App crashed when canceling video import

[fz0000]

Describe the bug
App crashed when canceling video import.

To Reproduce
Steps to reproduce the behavior:

1. Go to Editor.
2. Click on File -> Load and select any video file.
3. Click Cancel button on "Import frames from video" dialog
4. See error

Expected behavior
Go back to Editor window.

Screenshots

Desktop (please complete the following information):

• OS: [e.g. Windows 11]
• Version [e.g. 2.43.2]

Additional context

ScreenToGif/ScreenToGif/Windows/Other/VideoSource.xaml.cs

Lines 109 to 114 in 7432dcb

	private async void CancelButton_Click(object sender, RoutedEventArgs e)
	{
	await _viewModel.Cancel();
	DialogResult = false;
	}

It looks like the if (IsLoaded) check is missing before setting DialogResult = false, now that Cancel() has been updated to async.