Runtime Error

After a system rebuild I have re-installed Library Creator XML but every time I try to make a library it shows

Run-time Error '91':
Object variable or With block variable not set​

Can anyone help? What does it mean? :)
Well I've been offline for a while with HDD failure lost everything :mad:, so I'm starting basically from scratch. Luckily I had all the models backed up so I'm just putting it all together again. But this time everythings getting backup :)