Not exactly sure why ff2_witchdoctor throws such errors, the specified function that gets reported has required checks that should prevent them. Another thing to mention: the lines that get reported do not match the provided source code. It may be possible that you're running an older version which does not have the check.
Solution: either try to compile the source you attached and see what happens when you use it, or use ff2_death that is almost an exact copy(judging by it's source code). I run that subplugin and use it's abilities as well, and I get no errors.
Go to
https://forums.alliedmods.net/showpo...6280?p=2306280 and grab ff2_death.ff2 from the bottom of the post. Ability configs should be the same, just rename "plugin_name.".