What OS and version are you testing on? HTML Viewer relies on the system heavily, so you might be affected by SDK changes.
I notice your url starts with http and is not secure. This does not fly on newer macOS, everything must be https or you have to add a special entitlement to your app. If you are affected by this, the HTML Viewer is not loading the movie. You can use something like LittleSnitch or Charles to detect and monitor network activity.