Critical add file bug (images)

If you first add an image to your timeline you can't add any new/additional files using "after last file" which is now the default option. Same with existing timelines where the "last file" on the timeline is an image. I guess this is because of the missing end time. We had this before but it was probably re-introduced by changing "after last file" to the default add-file-method. Should be fixed as soon as possible (@maximilian.mittermei).