[Bug] App crashed when canceling video import

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 https://github.com/NickeManarin/ScreenToGif/blob/7432dcb468dcdf4cd05efdce0f4d02ff6644f235/ScreenToGif/Windows/Other/VideoSource.xaml.cs#L109-L114

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