I've just tested it and the same problem occurs with me (v2.0.9).
This is probably just a temporary problem. You should create a new issue. So you can best be helped.
Here is a quick hack:
Open the file /usr/bin/pencil
in an editor:
sudo nano /usr/bin/pencil
and replace content with the following code:
#!/bin/sh
APP_INI="/usr/share/evolus-pencil/application.ini"
if [ -x /usr/bin/xulrunner ]; then
/usr/bin/xulrunner "$APP_INI" "$@"
elif [ -x /usr/bin/iceweasel ]; then
/usr/bin/iceweasel --app "$APP_INI" --no-remote "$@"
else
/usr/bin/firefox --app "$APP_INI" --no-remote "$@"
fi
You can also build it yourself though, if you do not want to install the Firefox extension (Currently the latest version is 2.0.9):
wget https://github.com/prikhi/pencil/archive/v2.0.9.tar.gz
tar -xvf pencil-2.0.9.tar.gz
cd pencil-2.0.9/build
./build.sh linux
Start with:
xulrunner Outputs/Linux/application.ini
or
firefox --app Outputs/Linux/application.ini --no-remote