Another way to fix #697

[DarthGandalf]

No description provided.

[Photon89]

I had another look at it today. With the system call I get the following behavior:

1. The first time that apply_effect() is executed (when launching the plugin dialog), it runs without error.
2. The second time that it is executed (by pressing the reload or save button), the system call silently fails.
3. It still works in the first run and fails on subsequent ones when all the variables in the argument are substituted by fixed values (besides of $tmpfilename).