Beginner question about nil access

[fishsticks]

Hey all,

I very new to Nim and was writing a tiny command line todo application to get 
my bearings. [Here's a link to a code 
snippet](https://gist.github.com/anonymous/d2c2e8df2573d5a7be35f56caf8d3780) 
When I run this code, I get this error and I'm not sure how to circumvent it:
    
    
    Traceback (most recent call last)
    todo.nim(17)             todo
    todo.nim(11)             formatTodos
    SIGSEGV: Illegal storage access. (Attempt to read from nil?)
    

Any help would be much appreciated, thanks!