Plugins throw an error

Greetings,

I am new to Sigil. I have been trying to install and run some plugins, and they do not work. The plugins appear to install OK, and they appear in the plugins menu, but when I try to run them I get an error that says

I installed the testplugin, and it appears in the menu Plugins=>Edit=>testplugin

The plugin is installed in the directory

I am running Sigil 0.9.5 on Linux Mint 18.1

Anybody know how to fix this?

Thanks,
Banjo
(_)=='=~