Review application behavior for null file, or unparseable file

The last time I was running cone in an iPhone Simulator, I think I saw CPU usage when the app showed loading spinner with file as `null`. Maybe the fix could become obvious, but I should try to be a little bit systematic in how I discover this. I'm not sure if there's a way to automate a test for this.

There may be similar behavior for a file that cone can not parse, but I haven't researched yet. I notice that the loading spinner will appear as it apparently attempts to parse.

Start by reproducing the loading-spinner behavior in the iPhone Simulator with a null file and then with an unparseable file. Trace the behavior around parsing and loading, and determine whether an automated test is possible; done means the problematic spinner and CPU behavior are addressed and the relevant behavior is covered.

Written by the indexing model from the issue text.