Suggestion: don't process video if Project argument is not set or invalid

[dany123]

I am in the process of making an UI for the Shotcut application and I noticed that when there's no argument supplied to the Project parameter (so when the user would first add the filter to a video) the output is garbled - see screenshot # 2 (Edit: noticed that this part happens only if the aspect ratio of the video isn't 16x9).

If the argument is supplied but invalid (like "x"), the video is partially shown by cropping a bit of the top (see screenshot # 3, this was a 4x3 2.7k source in a 16x9 1080p timeline, but also happens for 16x9 in a 16x9) so the result is better, but not ideal.

I guess this could be some sort of intentional detail to tell the user he has to pay attention to the filter and add the project but my first thought was "oh, there's a bug/corruption, let me close it and try again".
Also had a look at the Kdenlive behaviour and it is the same.