Previewing a clip with no audio crashes the preview dialog

[owickstrom]

Describe the bug Previewing a clip with no audio clips in the selected sequence or parallel crashes the preview dialog.

To Reproduce Steps to reproduce the behavior:

1. Create a parallel with only video clips
2. Press the space bar
3. There should be a blank preview dialog and the program should be stuck

Expected behavior It should not let you preview if not supported, and as a minimal measure the preview functionality should handle exceptions better.

Desktop (please complete the following information):

• OS: Ubuntu 18.04
• Git: eae8f28
• Build environment: Nix shell + Cabal new-build on Ubuntu