Issue description

When you start the Title generator, lot's of dialogs appear like "Bar_1.svg is not a valid image file.", then an empty window appear.

Steps to reproduce

1. open OpenShot
2. click on Title -> Title

Technical details

• system: "x86_64-linux"
• host os: Linux 4.9.70, NixOS, 17.09.2430.ed3b1bc8448 (Hummingbird)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 1.11.16
• channels(root): "nixos-17.09.2430.ed3b1bc8448"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs
• Openshot: 2.3.4