Fix parameter name and check for reference file on bones

[LKAMinco]

merge_group.py and skinned_mesh.py fix using wrong parameter on inherited class init.

export reference file fix, when function has been executed on bones

[StjerneIdioten]

I will squash this and fix the commit message since it is kind of an urgent fix. But could you in the future try and keep your commit messages in this format. It makes my life a lot easier 😄 The repository is also commitizen friendly if you want to use that tool to help with it